The difference between a stiff and flexible tennis racket refers to the amount of flex or bend the racket has when it strikes the ball.
Stiff rackets are less flexible and tend to be more powerful and provide more control. They have a higher stiffness rating, which means they don't bend as much when the ball strikes the strings. They also tend to be heavier than flexible rackets, which can provide more stability and power.
Flexible rackets, on the other hand, are more pliable and have a lower stiffness rating, which means they bend more when the ball strikes the strings. This can provide more comfort and help absorb shock and vibrations. Flexible rackets also tend to be lighter than stiff rackets, which can allow for more maneuverability and faster swing speeds.
The choice between a stiff or flexible racket depends on the player's personal preference, playing style, and level of play. Generally, beginner and intermediate players may benefit from a more flexible racket, while advanced players who generate their own power may prefer a stiffer racket for more control.