What is the depletion region in pn junction?

Depletion region is the small region in the vicinity of the p-n junction which has been depleted of free charge carriers ( electron and holes) and has only immobile ions.

This happens when electrons in the n side and holes in the p side move towards  the junction and recombine.

This creates a depletion of charge carriers in the vicinity of the junction. And as a result atoms near the junction become ions.

Electric field is generated because of these immobile ions and opposes any further flow of majority charge carriers i.e. electrons in the n side and holes in the p side to cross over the junction. And hence, a potential barrier is created at the junction.