Police officer is a term used in some nations such as the United States that refers to a government recognized occupation of an upholder of the law. The more common term is now law enforcement officer (LEO). In the United States they are armed with firearms and have the power to arrest lawbreakers or to give tickets for more mundane issues such as traffic infringement.
While the term police officer is not universal, the governments of most nations have some occupation that performs a similar function. Not all police forces are armed with firearms. For example, those in Great Britain are not.