Psi is a protoss gameplay counter limiting the number of protoss units. The counter (available psi) is increased by constructing pylons and the maximum is 200. New units may be produced so long as the cumulative psi "cost" of all active units does not exceed the available psi.
The 200 point limit may be exceeded through the dark archon's mind control ability; this is legal and no new units may be built until consumption falls back below 200. The counters for supply, psi, and control are kept separately; a 600 point force may be constructed by capturing SCVs and drones and building up the non-protoss tech trees.