The One who knows the truth of things.
Allah is Al-Khabeer. The Knower of reality, The One whose knowledge is comprehensive, knowing the qualities and meanings of all things. He knows and understands the most hidden secrets. He is All-Aware and All-Acquainted. The One who knows the truth in every situation.
From the root kha-ba-ra (خ ب ر), which has the following classical Arabic connotations: to know, or be aware of the real inner nature of something, to understand the inner, the intrinsic nature of something, to know the reality of something, to know the secret inner state of something, to prove, try, test by experience (resulting in inner knowledge).
In modern Arabic, if you call someone خبير (khabir) it means he's an expert, i.e., possessing great knowledge about that particular field or subject.
This name often appears in pairs with al-Lateef (The All-Subtle) and al-Alim (The All-Knowing). The name is mentioned 45 times throughout the Qur'an. To be fully acquainted requires having both knowledge and knowing subtleties. Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ not only knows our actions, but He knows the state of our hearts. He is acquainted with the outwardly and inwardly.
Yaaa ayyuhal lazeena aamanut taqul-laaha; waltanzur nafsum maa qaddamat lighadiw wattaqual laah; innal laaha khabeerum bimaa ta’maloon
"O you who have believed, fear Allah. And let every soul look to what it has put forth for tomorrow – and fear Allah. Indeed, Allah is Acquainted with what you do." — (Qur'an 59:18)
Laa tudrikuhul absaaru wa Huwa yudrikul absaara wa huwal Lateeful Khabeer
"Vision perceives Him not, but He perceives [all] vision; and He is the Subtle, the Acquainted." — (Qur'an 6:103)
In tad’oohum laa yasma’oo du’aaa’akum wa law sami’oo mas tajaaboo lakum; wa Yawmal Qiyaamati Yakfuroona bishirkikum; wa laa yunabbi’uka mislu khabeer (section 2)
"If you invoke them, they do not hear your supplication; and if they heard, they would not respond to you. And on the Day of Resurrection they will deny your association. And none can inform you like [one] Acquainted [with all matters]." — (Qur'an 35:14)
Innal laaha ‘indahoo ‘ilmus saa’ati wa yunazzilul ghaisa wa ya’lamu maa fil arhaami wa maa tadree nafsum maazaa taksibu ghadaa; wa maa tadree nafsum bi ayyi ardin tamoot; innal laaha ‘Aleemun Khabeer (section 4)
"Indeed, Allah [alone] has knowledge of the Hour and sends down the rain and knows what is in the wombs. And no soul perceives what it will earn tomorrow, and no soul perceives in what land it will die. Indeed, Allah is Knowing and Acquainted." — (Qur'an 31:34)
Allah's سُبْحَٰنَهُۥ وَتَعَٰلَىٰ knowledge and awareness of all things is not from observation. Rather everything known is from the knowledge of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ. This is what is meant by the perfection of attributes. He possesses these qualities intrinsically, and they are His essence. He was al-Khaliq before the first creation was made. He was ar-Raheem before the creation of Adam (as). This is why scholars have mentioned that all of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ actions are derived from His most glorious and perfect names. For us, our names are derived from our actions. We have to prove and qualify ourselves to be called honest, trustworthy, or strong. Someone says I am strong, and we say, okay, pick this heavy thing up. We could infer from their muscles that they are strong, but that's only due to the weight training they had to do first. Whereas the qualities and attributes of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ just are and always have been. They are eternal and part of His being. The actions which come forth are an effect of having these names.
This name spurs the need to examine oneself. Just as Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is all-acquainted, we must ask ourselves if we are aware of what goes in ourselves. A level of honesty is necessary to unearth some truth; otherwise, what lies underneath remains hidden. What is it that motivates you? Are you too preoccupied with worldly pursuits? People like to think of themselves favorably and are over-confident in desirable traits (e.g., kindness, generosity, intelligence, disciplined, etc.), but would the world around you say the same?
Understand, the brain and body seeks to preserve, your ego wants to protect you from taking any punches and getting bruised. All this happens without our conscious knowledge, and so in a sense, we can be easily deluded. It takes constant examination to know this game of self-deceit and to understand the tricks of the brain before we can say we're "acquainted" or "totally aware." We have to prove to others and to ourselves whether we qualify to be worthy of being called by a certain title or name.