جَلَّاهَا (displays it): To show and reveal it.

This happens when the day lights in its duly time. The subject (it) in this verse is the day that clears the darkness of the night, so that, it shows and reveals the things which are on the earth, the pronoun (it) here goes back to the darkness of the night. T

The subject, on the other hand, could be Allah (swt), meaning, Allah (swt) is the One who displays it.