Allah says,

﴿إِذَا السَّمَآءُ انشَقَّتْ ﴾

(When the heaven is split asunder,) This refers to the Day of Judgement.

﴿وَأَذِنَتْ لِرَبِّهَا﴾

(And listens to and obeys its Lord) meaning, it listens to its Lord and obeys His command to split apart. This will occur on the Day of Judgement.

﴿وَحُقَّتْ﴾

(and it must do so.) meaning, it is right for it to obey the command of its Lord, because it is great and cannot be rejected, nor overcome. Rather it overpowers everything and everything is submissive to it. Then Allah says,

﴿وَإِذَا الاٌّرْضُ مُدَّتْ ﴾

(And when the earth is stretched forth,) meaning, when the earth is expanded, spread out and extended. Then He says,

﴿وَأَلْقَتْ مَا فِيهَا وَتَخَلَّتْ ﴾

(And has cast out all that was in it and became empty.) meaning, it throws out the dead inside of it, and it empties itself of them. This was said by Mujahid, Sa`id, and Qatadah.

﴿وَأَذِنَتْ لِرَبِّهَا وَحُقَّتْ ﴾

(And listens to and obeys its Lord, and it must do so.) The explanation of this is the same as what has preceded.