That guy on the field in black and white stripes? He's a referee, an official in a sport like football or basketball who enforces the rules.
In sports, referees are there to make sure the rules are followed and everyone gets a fair shake. This word works as both a noun and a verb: referees referee games. Off the court, the world is full of everyday referees. A court appointed lawyer who investigates and reports on a case is a referee. And academics who evaluate each other's work are referees. In all cases, referees get to make official calls on what's OK and not OK.