Psionic Matrix refers to the circular matrix that extends outwards from a Protoss Pylon. Protoss buildings can be warped in only within this circle. It also provides power to the Protoss buildings once warped in, allowing them to produce units and research upgrades. The buildings that do not require the Psionic Matrix either to be built or to operate are the Nexus, Assimilator, and Pylon.
An example of strategic Pylon placement is in the construction of Photon Cannons, such as when performing a Cannon Rush or setting up base defences; the Protoss player must first warp in a Pylon, and only then can Photon Cannons be warped in. Having key areas of the base already covered by the Psionic Matrix can save time if Cannons are needed—for example, around the Mineral Line if an incoming Mutalisk harass is scouted, at a choke or ramp if Dark Templar are expected.
The Psionic Matrix extends across elevation: buildings can be warped in using power from a Pylon that is on a different level from them, for example, the Pylon can be built on high ground in the main base and Gateways or Cannons can be warped in on the low ground outside.