It is a tool that measures how fast someone is driving. It is a gun shaped metal block that has a screen on it that tells how many miles per hour a vehicle is going by using green digital numbers.
Role in episode
SpongeBob tattles on Patrick for going slightly over the speed limit, but since Patrick stops immediately, the police merely warns SpongeBob not to tattle, instead of doing anything to Patrick. The meter gun is a key item in the Bikini Bottom Police Department, and is used to check the speed of boat mobiles.