This is a term used in many sports, such as football, as a label for the game official monitoring action on the field or court, whichever the case may be. In the case of football, there are many "referees" on the field during the game. Furthermore, in football, only one of the official's titles is referee, the rest of the officiating crew is either known as linesman or judges. In total, the NFL has seven officials on the field. Their responsibilities vary, but they all call penalties and watch to be sure everything in their purview is legal. The goal of the officials is to ensure the game runs smoothly and according to the rules.
The term referee is commonly shortened to "ref." In football, the referee is sometimes known as the lead referee, head umpire, or head referee. He is tasked with overseeing the action on the field and is the chief official for the game. They are also the one who makes the penalty calls publically so coaches, fans, and players can hear the call.