This was asked awhile ago so I'm sure you've found an answer by now but I'll answer anyway.
A psi ball is a ball of energy. Basically, you use the energy in your body to form it; Usually by putting your hands close together and focusing your energy to form a ball in-between them. I'd say that it's probably used as a test to determine your energy control but there could be a different reason.
I have no idea if it's real or not ( I've never been able to form one ) but it would probably take a lot of energy and focus to pull it off.