A fair ball is a ball that has been hit, and stays in-bounds. There are chalk lines that mark the out of bounds There are also tall poles in the outfield that help in telling whether a long fly ball is fair or foul, these are called foul poles. The umpires will call whether a ball is fair or foul. If a fly ball lands in the infield between home and first base, or home and third base, and then bounces to foul territory without touching a player or umpire and before passing first or third base, it is a foul ball. You often see a player bunt the ball and then the defensive players don't touch the ball but wait for it to roll out of bounds. This can be kind of a risky play because the ball might not roll out. If a ball settles on foul territory or is touched by a player in foul territory, it is a foul ball. If a fly ball hit the ground on or beyond first or third base and then bounces to foul territory, it is a fair ball.