What is the purpose of a whistle in a football game?

Answer 1

The whistle is mostly used by the referee to signal players to stop or start play. That can be for a number of reasons. It can be used to award free kicks, corners, and penalties. It is blown to tell players to begin play at the kickoff and to end play, with two whistles at half time, and three distinct whistles at the end of the match.

If players get aggressive referees can blow the whistle in short sharp bursts to get players to 'break it up'. After a free kick or corner is awarded the whistle is blown again, to tell the kicker he is allowed to begin the set play.