When you finish a hole of golf in one under par. For instance, if the score card says you should make a five on hole if you're a good golfer, and you make four, it's a birdie. It usually pays double on the bet for the hole, and betting on golf is an absolute part of the game. People who play golf should always bet when they play; that's why there is a golf handicap system based on your last 20 scores.