Psychic boosters (a.k.a. psionic boosters) are protoss devices installed on capital ships to allow vessel-wide psionic communication. It is through these that protoss commanders can send telepathic messages on a wide scale in a manner similar to a terran radio, able to sense the recipients' reaction(s) to his/her message. Broadcasts from the booster are received by psychic communication loops, which are interwoven into the hulls of protoss ships.
It appears that portable psychic boosters exist also, carried by protoss commanders when not aboard their flagship. These devices can send messages throughout an entire fleet simultaneously and cannot be intercepted by enemy forces.