The main reason for humanity existence

This World was created for a Wisdom

Here Allah tells us of His justice, and that He is far above mere play, folly and falsehood. This is like the Ayah:

(And We created not the heaven and the earth and all that is between them without purpose! That is the consideration of those who disbelieve! Then woe to those who disbelieve from the Fire!) (38:27)

(Did you think that We had created you in play, and that you would not be brought back to Us So Exalted be Allah, the True King: La ilaha illa Huwa, the Lord of the Supreme Throne!) (23:115-116) Then Allah says:

(Verily, the Day of Judgement is the time appointed for all of them) This is the Day of Resurrection, when Allah will judge between all creatures, and He will punish the disbelievers and reward the believers.

(is the time appointed for all of them) means, He will gather all of them, the first and the last of them.

(The Day when a near relative cannot avail a near relative in aught,) means, no relative will be able to help another relative. This is like the Ayah:

(Then, when the Trumpet is blown, there will be no kinship among them that Day, nor will they ask of one another.) (23:101)

(And no friend will ask a friend (about his condition). Though they shall be made to see one another) (70:10-11) which means, he will not ask his brother about how he is, even though he can see him with his own eyes.

(and no help can they receive,) means, no relative will help another, and no help will come to him from outside.

(Except him on whom Allah has mercy.) means, and nothing will be of any avail that Day except the mercy of Allah towards His creation.

(Verily, He is the All-Mighty, the Most Merciful.) means, he is the Almighty, with immense mercy.

Tafsir Ibn Kathir