This is the term used to describe medial rotation of the radius, a simple twist of the forearm at the radioulnar joint. In the anatomical position, this action turns the palm inward. In pitching, this action generally turns the palm forward and away from the body at pitch release.
Because the pronator teres attaches the radius to the medial epicondyle of the humerus, the secondary muscular action of pronation is elbow flexion. This secondary action helps prevent potential damage from forearm flyout. By decelerating elbow extension, pronation helps prevent the olecranon of the ulna from slamming into the olecranon fossa of the humerus.
Additionally, powerful pronation through the release of the ball can actually add velocity to the pitch.
See also: supination.