The word police refers to the members of a law enforcement organization, or to the organization itself. If you see a crime being committed, you should call the police.
The job of the police is to protect citizens and enforce laws. You might see police patrolling parks or maintaining order at large gatherings, like concerts and demonstrations. As a verb, to police is to enforce laws or keep a watchful eye on things. Actual police officers might police a political protest, while teachers are sometimes called upon to police school dances and parties.