Definition: Fish is a betting game for groups of golfers that is modeled after a popular fishing bet, hence its name. The fishing bet is threefold: a bet on who catches the first fish, a bet on who catches the biggest fish, and a bet on who catches the most fish.
Likewise, Fish on a golf course is threefold, but this time involving birdies: a bet on who makes the first birdie, a bet on who makes the longest birdie, and a bet on who makes the most birdies. Each of the three bets carries its own dollar amount (usually the same for all three bets).
Keep track of the distance of each birdie in case you need to go to a tiebreaker for most birdies. The tiebreaker is greater total distance on the birdies. Also, eagles can be set to be worth two "fish" (birdies) for the most birdies bet.