A PSI Pop gives you a miniscule amount of PSI Energy (denoted by the brains in the top left corner of the HUD) through use. On the PS4/PS5, if you have one, you'll be urged to press
until it goes away. PSI Pops cost 50 Psitanium in the Otto-matic, and for the amount of energy they recover, are usually not worth it unless you're in a boss fight.