A referee, or "ref" for short, is a person whose job is to ensure that the rules of a competitive event are observed and obeyed, or at least in theory. In reality, they are the scapegoats for any shortcomings that the players have and are generally treated like utter shit by both players and spectators alike.
Soccer Player: Dude, that referee was such a fucking piece of shit. Hey, nice call, fucker!
Person: Shut up. How would you like to try to manage a fair game, only to have half of the field scream vulgarities and death threats at you, then vandalize your car?
Soccer Player: But...but...it was a bad call.
Person: Damn, you're a fucking dolt.