Psychic communication loops are broad spiderweb transceivers woven into the hulls of protoss ships. It is with these devices that protoss craft are able to receive telepathic messages from psychic boosters or other sources.
References
Mesta, Gabriel (July 1, 2001). StarCraft: Shadow of the Xel'Naga. Simon & Schuster (Pocket Star). ISBN 0-671-04149-5