A Photon is a subatomic light wave/particle.
A Cannon is a device that shoots something.
Thus a Photon Cannon is a device that shoots Light.
Protoss Player <to all>: "HAHAHAHA! my dragoons just pwned ur hydras like there all photophobic! ROFLMA"--NUCLEAR LAUNCH DETECTED.--"OH SHIT!"
Zerg Player <to Terran ally>: "gj"
Terran Player <to Zerg ally>: "thx hehe >:)"