The Last, The End, The Endless.
Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is Al-Akhir (in Arabic: ٱلْآخِرُ), The One who’s existence will extend past the end of this universe. He will be the last just as He is the first.
NOTE: If you are reading the 99 names of Allah guide in order, then much of what's been written here is the same as Al-Awwal as these names have been explained in pairs. The next name in this series is az-Zahir.X
From the root a-w-l which has the following classical Arabic connotations: to be first, principle, to be before, to come back to, to return, to precede, to be the first part, to be foremost.
The understanding and belief in the name establishes tawheed (oneness of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ). Al-Awwal means He was first, or is the first. But this does not mean that there was nothing before. Rather the beginning was Him. He exists without a beginning and is the source from which everything comes into existence.
Al-Awwal also implies an opposite, if there is a first, then there must be a last, and this is the name Al-Akhir. This does not mean that Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ has a beginning and an end. Instead, it tells us He is the start, and He is the end. It's the race of life; we start with Him and return to Him. Some may take shortcuts only to get lost, and the wiser pace themselves as they know it's a marathon. A few may try to trip or sabotage others to gain an advantage, but they fail to see the race is within themselves. There is no first or second-place prize. At the end of the journey, the satisfied will be those who gave their best effort; they struggled beautifully and gave it their all. For them will be the prize of the akhirah. You don't want to have reached the end of the race to realize you had much more to offer; you want to be tired at the end.
Huwal Awwalu wal’Aakhiru waz Zaahiru wal Baatinu wa huwa bikulli shai’in Aleem
"He is the First and the Last, the Ascendant and the Intimate, and He is, of all things, Knowing." (Qur'an 57:3)
Allazeena izaaa asaabathum museebatun qaalooo innaa lillaahi wa innaaa ilaihi raaji’oon
"Who, when disaster strikes them, say, 'Indeed we belong to Allah, and indeed to Him we will return.'" (Qur'an 2:156)
It is narrated on the authority of Abu Huraira that the Messenger of Allah ﷺ said: Men will continue to question one another till this is propounded: Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ created all things but who created Allah? He who found himself confronted with such a situation should say: I affirm my faith in Allah. 
To those with a curious minds, we can't fathom how something can exist from nothing. We're so ingrained by our experiences as to seeing how one thing gives birth to another that we ask the same question of God. But as Prophet ﷺ tells the companions who ask Him this question, he replies that it's a matter of faith. The Qur'an addresses the hypocrites and the disbelievers in many verses. For example:
Awa lam yakfihim annaaa anzalnaa ‘alaikal kitaaba yutlaa ‘alaihim; inna fee zaalika larahmatanw wa zikraa liqawminy yu’minoon (section 5)
"And is it not sufficient for them that We revealed to you the Book which is recited to them? Indeed in that is a mercy and reminder for a people who believe." (Qur'an 29:51)
The human mind has many limitations, and we're often fooled by ourselves. We don't know what we don't know, and we can't know the limits of how our intelligence may be bounded. A lot of psychological work has been done to show the many ways in which we're unreliable narrators. One such fallacy is the illusion of explanatory depth (IOED). If you look at the simplist of things we use every day and try to explain in detail how they work, we quickly realize that much of our knowledge is surface level. For example, what's a toilet? We likely answer with confidence what seems to be a silly question. Of course, we know what a toilet is and how to operate one, but when you press the flush button, what's really happening underneath the tank? That's what makes the toilet a toilet. We are often unaware of our ignorance, yet we ask irrelevant questions of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ that we cannot comprehend. And so the answer Prophet Muhammad ﷺ gives may be the most intelligent answer.
There is an amazing du'a the prophet ﷺ used to make before going to bed, which asks for debts and desires to be cleared so he may rest in peace. And in His du'a, it calls upon these two names Al-Awwal and Al-Akhir.
اللَّهُمَّ رَبَّ السَّمَوَاتِ وَرَبَّ الأَرْضِ وَرَبَّ الْعَرْشِ الْعَظِيمِ رَبَّنَا وَرَبَّ كُلِّ شَىْءٍ فَالِقَ الْحَبِّ وَالنَّوَى وَمُنْزِلَ التَّوْرَاةِ وَالإِنْجِيلِ وَالْفُرْقَانِ أَعُوذُ بِكَ مِنْ شَرِّ كُلِّ شَىْءٍ أَنْتَ آخِذٌ بِنَاصِيَتِهِ اللَّهُمَّ أَنْتَ الأَوَّلُ فَلَيْسَ قَبْلَكَ شَىْءٌ وَأَنْتَ الآخِرُ فَلَيْسَ بَعْدَكَ شَىْءٌ وَأَنْتَ الظَّاهِرُ فَلَيْسَ فَوْقَكَ شَىْءٌ وَأَنْتَ الْبَاطِنُ فَلَيْسَ دُونَكَ شَىْءٌ اقْضِ عَنَّا الدَّيْنَ وَأَغْنِنَا مِنَ الْفَقْرِ
Suhail reported that Abu Salih used to command us (in these words): When any one of you intends to go to sleep, he should lie on the bed on his right side and then say: "O Allah. the Lord of the Heavens and the Lord of the Earth and Lord of the Magnificent Throne, our Lord, and the Lord of evervthing, the Splitter of the grain of corn and the datestone (or fruit kernal), the Revealer of Torah and Injil (Bible) and Criterion (the Holy Qur'an), I seek refuge in Thee from the evil of every - thing Thou art to sieze by the forelock (Thou hast perfect control over it). O Allah, Thou art the First, there is naught before Thee, and Thou art the Last and there is naught after Thee, and Thou art Evident and there is nothing above Thee, and Thou art Innermost and there is nothing beyond Thee. Remove the burden of debt from us and relieve us from want." Abu Salih used to narrate it from Abu Huraira who narrated it from Allah's Apostle (ﷺ)