One of the 7 ways to reach base without a hit in professional baseball, a fielder's choice can only occur if there is a situation where a runner must run. The idea is that the fielders want to get the lead runner out, to prevent runners from entering scoring position.
The most common fielder's choice is when the batter hits the ball in play, the first-base runner heads towards second, and the ball is thrown to second base by an infielder. However, there is not enough time for a double play, so the batter reaches first safely. This is scored as an out, not a hit, for the batter: the batter is responsible for the lead runner being out, and the team usually gains no advantage. Note that is possible to bat in a run using a fielder's choice, if there are runners on the corners, and the third base runner heads home.