It is derived from the word infatarat in the first verse. Infitar is an infinitive which means to cleave or split asunder, thereby implying that it is the Surah in which the splitting asunder of the sky has been mentioned.
This Surah and the Surah At-Takwir closely resemble each other in their subject matter. This shows that both were sent down in about the same period.
Its theme is the Hereafter. According to a tradition related in Musnad Ahmad, Tirmidhi, Ibn al-Mundhir, Tabarani, Hakim and Ibn Marduyah, on the authority of Hadrat Abdullah bin Umar, the Holy Messenger (upon whom be peace) said: "The one who wants that he should see the Resurrection Day as one would see it with one's eyes, should read Surah At-Takwir, Surah Al-Infitar and Surah Al-Inshiqaq."
In this Surah first the Resurrection Day has been described and it is said that when it occurs, every person will see whatever he has done in the world. After this, man has been asked to ponder the question: "O man, what has deluded you into thinking that the God, Who brought you into being and by Whose favor and bounty you possess the finest body, limbs and features among all creatures, is only bountiful and not just? His being bountiful and generous does not mean that you should become fearless of His justice."Then, man has been warned, so as to say: "Do not remain involved in any misunderstanding. Your complete record is being prepared. There are trustworthy writers who are writing down whatever you do." In conclusion, it has been forcefully stated that the Day of Resurrection will surely take place when the righteous shall enjoy every kind of bliss in Paradise and the wicked shall be punished in Hell. On that day no one shall avail anyone anything. All powers of judgment shall be with Allah.
Source: Sayyid Abul Ala Maududi - Tafhim al-Qur'an - The Meaning of the Quran
وَإِنَّ عَلَيْكُمْ لَحَافِظِينَ
إِنَّ الْأَبْرَارَ لَفِي نَعِيمٍ
وَإِنَّ الْفُجَّارَ لَفِي جَحِيمٍ
يَصْلَوْنَهَا يَوْمَ الدِّينِ
وَمَا هُمْ عَنْهَا بِغَائِبِينَ
وَمَا أَدْرَاكَ مَا يَوْمُ الدِّينِ
ثُمَّ مَا أَدْرَاكَ مَا يَوْمُ الدِّينِ