This question deals with two types of energy that go hand-in-hand; kinetic and potential energy.
Potential energy is energy that is stored inside a body at a position relative to others, that will eventually be used at some point. Kinetic energy is energy that is possessed by a body while being in a state of motion.
In the example, the baseball player has potential energy stored, of which he will use to throw the ball. When he "winds up" (gets ready to throw the ball), this potential energy is transformed into kinetic energy, and is transferred to the baseball. Once the baseball leaves his hand, it's potential energy is converted to kinetic. Once it reaches the player at first base, the kinetic energy converts back to potential as it slows down and eventually comes to rest in the glove of the player, transferring that energy to him.
