Psionic Matrix is a term that refers to the circular matrix that extends outwards from a Protoss Pylon or from a Warp Prism in Phasing Mode. The only place where most Protoss buildings can be warped in is within this Matrix. The Matrix also provides power to Protoss buildings, allowing them to produce units and research upgrades. Buildings that do not require a Psionic Matrix to be warped in and function are the Nexus, Assimilator, and Pylon. Areas on the map covered by Psionic Matrices are the only locations where Warpgates can warp in units on demand.
There are two differences between the Psionic Matrices generated by Pylons and Warp Prisms: the radius, and how the terrain affects the Psionic Matrix. Pylons cover a radius of 6.5, while Warp Prisms in Phasing Mode only provide the Matrix over a 3.75 radius. In Wings of Liberty, Pylons on the low ground provide power to the high ground, but in Heart of the Swarm the Psionic Matrix no longer covers the high ground. If the Pylon is on the high ground, it still provides power to the low ground. Warp Prisms, however, are unaffected by the change - i.e. they still provide the Psionic Matrix radius regardless of the terrain.