The Ramban is referring to Akabyah ben Mahalalel who says that to keep oneself on the righteous path one should remember that he will eventually have to answer for his actions in the heavenly court, and that he cannot hide is actions from Hashem, since he is everywhere and knows everything.
The Vilna Gaon asks: why does he say that there will be both judgement and accounting?
- Judgement refers to any transgressions a person committed.
- Accounting refers to wasting ones time and efforts and misusing them for sin instead of mitzvot.
The Ramchal says: And when he contemplates further and pictures in his mind the moment he enters before the great Beit Din of the heavenly host, when he finds himself before the King of kings, the Holy One, blessed be He, who is absolutely pure and holy, in the midst of the assembly of holy ones, mighty servants, strong in power, obeying His word, without any blemish whatsoever, and he stands before them, base, lowly, and petty in and of himself, defiled and polluted due to his deeds. Will he then raise his head? Will he have what to answer?
And when they ask him: “where has your mouth gone? Where is the pride and honor which you assumed in your world?” What will he answer? What will he reply to this rebuke? Behold, certainly if for one moment, a person were to visualize in his mind this truth with a true and strong picture, all of his arrogance would blast off in flight, never to return.
(Adapted from A Letter for the Ages pages 50-52)