Psi or psionics is the source of power for the Protoss, derived from crystal technology.
It is equivalent to Terran supplies and Zerg control. Protoss Building usually need to be in a psi field to work. Without psi, Protoss can not warp in any units onto the battlefield. The oldest species known to weld it were the Xel'Naga and can be roughly measured in Psionics Ratings.