It is a stationary cannon that fires a blue energy beam. It is most likely capable of destroying anything.
Role in episode
Plankton, upset that the Chum Bucket might be destroyed, cries beside the sonic cannon, which causes it to activate and make a hole in the Chum Bucket wall. SpongeBob sees through the hole that Plankton is sad, and gives Plankton the idea to bribe the Bikini Bottomites so that they will sign a petition to keep the Chum Bucket and destroy the Krusty Krab.