Every racket is restrung to your desired tension. This is the tightness of the racket strings. Most tennis rackets are restrung within the 50-60lbs range, which you will find is the manufacturer's recommendation. The lower the tension the more power you will be able to generate with the 'trampoline effect' of the string when hitting the ball. On the opposite end of the scale a higher tension will provide more control with a bigger sweet spot across the racket head. You may choose to have a slightly different tension on the Mains strings (vertical) compared to the Cross strings (horizontal).
Squash rackets have a much lower tension range, usually the manufacturer's recommendation is between 20-30lbs. Any higher than this range and you are risking frequent string breakages and possible frame damage. The low tension range provides increased power and a higher string tension will give more control.
Badminton rackets are also restrung within the 20-30lbs range. As the rackets are much lighter and the strings are a lot thinner. The principle of power and control for lower and higher tension are the same.