The stiffness (or flexibility) of a racquet is a very important feature. In simple terms, this is how much the racquet bends when you hit the ball.
A very stiff racquet bends very little and more power is transferred to the ball.
A flexible racquet does not transfer as much power to the ball.
The flexibility is not often shown by the manufacturer. You will have to use another source to obtain this information.
The larger racquet retailers or tennis magazines can be a source of this information
also see Frame Profile page
The stiffness of a racquet is important, and you should select the model carefully.
One guide is the frame profile of your existing racquet. Measure this and compare it to your new racquet.
Increase this value and you should have a stiffer racquet. A thinner value would provide a more flexible racquet.