The Postponer, He Who Puts Far Away.
Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is Al-Mu'akhkhir (in Arabic: ٱلْمُؤَخِّرُ), The One who wisely delays or holds back. By His will, he can detain unbelievers and hold them back on the day of judgment.
From the root hamza-kha-ra (أ خ ر), which has the following classical Arabic connotations: to cause, to go back, backwards, retreat, recede, to retire, to drive back, to put behind or after, to cause, to remain behind, hang back, lag behind, to postpone, put off, or delay.
The names Al-Mu'akhkhir and Al-Muqqadim are two names in this series that are closely connected and should be mentioned together for complete comprehension. Al-Muqqadim is The Promoter, or the one who brings forth, and the other is Al-Mu'akhkhir, The Postponer, or the one who pushes away.
For this section, we will focus on what it means for Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ to be called by the name Al-Mu'akhkhir. You can also go back to the previous name, where we discuss Al-Muqqadim in more depth. The first interpretation is that Al-Mu'akhkhir is the one who pushes away whom He wills. Some of His creations are honored and promoted, like His angels, prophets, friends, and scholars. But the enemies of Islam He pushes away and banishes them. A clear example from the Qur'an would be the nation of 'Aad, "And that was 'Aad, who rejected the signs of their Lord and disobeyed His messengers and followed the order of every obstinate tyrant. And they were [therefore] followed in this world with a curse and [as well] on the Day of Resurrection. Unquestionably, 'Aad denied their Lord; then away with 'Aad, the people of Hud." (Qur'an 11:59-60)
Du'a from hadith book:
For the name Al-Muqqadim, no ayah in the Qur'an explicitly calls upon this name. Still, in the hadith corpus, we find a supplication that the Prophet ﷺ recites describing Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ as being "anta-l-muqaddim" and "wa anta-l-mu akh-khir."
Allahumma lakal-hamd. Anta qaiyyimus-samawati wal-ard wa man fihinna. Walakal-hamd, Laka mulkus-samawati wal-ard wa man fihinna. Walakal-hamd, anta nurus-samawati wal-ard. Wa lakal-hamd, anta-l-haq wa wa'duka-lhaq, wa liqa'uka Haq, wa qauluka Haq, wal-jannatu Han wan-naru Haq wannabiyuna Haq. Wa Muhammadun, sallal-lahu'alaihi wasallam, Haq, was-sa'atu Haq. Allahumma aslamtu Laka wabika amantu, wa 'Alaika tawakkaltu, wa ilaika anabtu wa bika khasamtu, wa ilaika hakamtu faghfir li ma qaddamtu wama akh-khartu wama as-rartu wama'a lantu, anta-l-muqaddim wa anta-l-mu akh-khir, la ilaha illa anta
"O Allah! All the praises are for you, You are the Holder of the Heavens and the Earth, And whatever is in them. All the praises are for You; You have the possession of the Heavens and the Earth And whatever is in them. All the praises are for You; You are the Light of the Heavens and the Earth And all the praises are for You; You are the King of the Heavens and the Earth; And all the praises are for You; You are the Truth and Your Promise is the truth, And to meet You is true, Your Word is the truth And Paradise is true And Hell is true And all the Prophets (Peace be upon them) are true; And Muhammad is true, And the Day of Resurrection is true. O Allah ! I surrender (my will) to You; I believe in You and depend on You. And repent to You, And with Your help I argue (with my opponents, the non-believers) And I take You as a judge (to judge between us). Please forgive me my previous And future sins; And whatever I concealed or revealed And You are the One who make (some people) forward And (some) backward. There is none to be worshipped but you" 
A helpful Qur'anic verse:
In the Qur'an, one verse refers to the words qaddama and wa akhkhar, which, if we analyze, can further help us understand the meaning of both names.
Yunabba ul insaanu yawma 'izim bimaa qaddama wa akhkhar
"Man will be informed that Day of what he sent ahead and kept back" — (Qur'an 75:13)
In this ayah, Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is instructing us to hasten to do good because on the day of resurrection, we will bring forth (qaddama) all of our deeds and they will testify to the good or bad that we've done. Next, it mentions (wa akhkhar), things we've left behind or kept back. This implies different meanings. Some commentators suggest it can mean leaving behind evil on earth, others interpret it in the affirmative, leaving behind a good legacy that perpetuates good for him even after he has returned to Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ (e.g., sadaqah jariyah - a perpetual, on-going charity). Allah's Messenger ﷺ said: He who introduced some good practice in Islam which was followed after him (by people) he would be assured of reward like one who followed it, without their rewards being diminished in any respect. And he who introduced some evil practice in Islam which had been followed subsequently (by others), he would be required to bear the burden like that of one who followed this (evil practice) without their's being diminished in any respect.
Trials from the life of the Prophets:
One thing that's important to understand is Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ doesn't just delay or demote things for quote-unquote 'bad people,' for lack of a better term. Some pious people are delayed in marriage or in having kids. Each person has their own fitnah (meaning trial or affliction) to the level Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ knows they're capable of shouldering. "And We will surely test you with something of fear and hunger and a loss of wealth and lives and fruits, but give good tidings to the patient," (2:155), and "Allah does not charge a soul except [with that within] its capacity." (2:286) We must recognize He is Aleemul Hakeem, the all-knowing, the all-wise and see that things don't just happen, they happen just (i.e., by the will of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ).
We can even briefly study the lives of the Prophets to understand how He delays even those closest to Him. For example, in the story of Prophet Ayyub (as) he lost everything in a great tragedy that killed all His children. In the account of Ibn Kathir:
Satan once again visited Ayyub disguised as sympathising bystander, and remarked with sorrow, "The circumstances under which your children died were sad. Surely, your Lord is not rewarding you properly for all your prayers."
Ayyub responded, "Allah gives me what is good for me, and takes away what Allah sees that is displeasing for Him and for me. Allah sees that all of these children may be a strong test & trial for me, so Allah took them away from me. Whether a thing is beneficial or harmful to me, I will remain firm in my belief and remain thankful to my Creator."
The same can be said for Prophet Nuh (as). Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ showed mercy, He gave people every chance to listen and seek His guidance, but only a few did. Prophet Nuh (as) had to endure 950 years of preaching to an unreceptive and at times, hostile crowd, "And We certainly sent Noah to his people, and he remained among them a thousand years minus fifty years, and the flood seized them while they were wrongdoers." (Qur'an 29:14) Even when the flood came from Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ a clear sign of their disobedience, Nuh (as) wife and son were still was among deniers and they drowned in the flood, "[But the son] answered: "I shall betake myself to a mountain that will protect me from the waters." Said [Noah]: "Today there is no protection [for anyone] from God's judgment, save [for] those who have earned [His] mercy!" And a wave rose up between them, and [the son] was among those who were drowned" (Qur'an 11:43)
From these two names, Al-Muqaddim, and Al-Mu'akhkhir, the believer understands whether Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ decrees for us promotion or delay we must not waver in our faith. Wallaahu khairul maakireen, "Allah is the best of planners."  We must accept His decree graciously and endure patiently, for there is wisdom in it. And if you have fallen back, do not despair, for Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is Al-Muqaddim, the one brings forward. This is why it's important to mention the names together. So as to not lose hope in Him. There is nothing that can't stop Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ mercy and to increase you in all aspects of living a good life. So never despair. As long as you are breathing Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ can improve your situation. As Prophet Ayyub said, "I will remain firm in my belief and remain thankful to my Creator."