A free ball is a post-foul situation in which the incoming player is snookered and cannot directly see both extreme points of the ball that is on. The referee decides after the balls have stopped.
The player's option
When a free ball is called, the player may nominate another ball to act as the ball on. It carries the nominated ball's value for that shot. A potted colour is normally respotted, while a red remains off the table under the frame rules.
Limits
The player must still contact the nominated ball first; a free ball does not rewrite the status of every other ball. If the situation is unclear, wait for the referee's call.
Rule wording can change, so use the current WPBSA edition for competition play.