Example: With a runner on first, the batter hits a ground ball to the shortstop. Instead of throwing the batter out at first, the shortstop chooses to throw to second base, making the runner out. The runner is called out, but meanwhile, the batter reaches first base safely. This play is a fielder's choice.
A batter who reaches base safely as the result of a fielder's choice is not credited with a hit; he is charged with an out as if he had been put out himself.