A cop is a police officer. If you realize you're speeding in a school zone, you might look around nervously, hoping you won't see a cop.
Cop is an informal, somewhat derogatory word for a police officer. When it's a verb, it means "to steal" or to "strike an attitude." An angry cop might say to a young troublemaker, "Hey, don't cop an attitude with me!" In the US, a legal defendant can also "cop a plea," or agree to a plea bargain. The earliest meaning of cop was "to catch," probably from the Latin capere, "to take."