This term can be used in two distinct ways. It may refer to the
action of a quarterback throwing the ball underhanded or
side-armed, though sometimes over-handed, usually a short distance,
to a running back or receiver.
It may also refer to the actual field of play. The latter definition is rarely used in the US. It is more common for an individual to refer to it as a field, rather than pitch. This definition is much more popular, and more widely used, outside the US. Within the US, the former definition is used almost exclusively. The rules of football require the quarterback be behind the line of scrimmage in order to throw the ball. If the intended receiver is close by, the "pass" is referred to as a pitch. If the play calls for a pitch, the receiver will usually also be behind the line of scrimmage. Furthermore, a pitch is usually made to the right or left side of the field; it is rarely a forward pass.
A play involving a pitch is often designed to catch the defense off guard by moving the ball to one side of the field. An example of a pitch play would involve the quarterback pretending to go back to pass the ball, but instead throws it underhanded to a receiver located about fifteen feet to the right. The receiver is then responsible for rushing the ball up-field.