Originally tennis rackets were made of wooden frames with strings from the intestines of animals. Not much changed until the late 1960's. Howard Head, an engineer, revolutionized the sport by designing a racket with a larger frame and one made of lightweight aluminum.
Due to the lower mass of the racket, a greater acceleration can be achieved from the same applied force. Balls hit from the lighter racket came off much faster. The strength of the new metal racket also meant less bending in the frame from the impact, resulting in more of the energy going to the ball and less to the frame.
Another great advantage of the Head Racket was that the larger frame meant a larger "sweet spot". A sweet spot is the place on the racket where the ball will get the maximum rebound power (energy per time) from the collision.