Form is a way of performing a movement to avoid injury and cheating.
Exercises or drills in sport have a recognized way of performing the movements that have two purposes:
By using proper or 'good' form, the risk of injury is lowered
Good form ensures that the movement only uses the main muscles, and avoids recruiting secondary muscles. As a muscle fatigues, the body attempts to compensate by recruiting other muscle groups and transferring force generation to non-fatigued units. This reduces the benefits in strength or size gain experienced by the muscles as they are not worked to failure.