String tension refers to how tight the racket string is tied to the racket, and it’s measured in pounds (lbs) normally. Badminton string on a racket is divided into the main and cross, where the main is the vertical string lines whereas the cross is the horizontal string lines. When you ask a stringer to string your racket at 25 lbs for example, the stringer normally will tie it at either 25X27 lbs (main X cross), or 24X26 lbs or 25X25lbs, depending on the stringer’s experience. There is no hard rule on this but for normal players, you do not need to be too skeptical on this, unless you know for sure what you tension you need for both cross and main. Otherwise just mentioning the single tension number will do.

The tighter you tie your string, the less bouncy your string becomes, which means less repulsion power will be generated from the string bed. However your shots will be more accurate and sharper, due to the smaller concentrated sweet spot and the string bed is less ‘bouncier’. As a general rule, you would need to have good strength if you wish to tie your string at 27 lbs or above. For average players, 22-26 lbs will be good enough. Over-loose string will cause the string bed to be too bouncy, and hence make the control of your shot execution harder.

On the other hand, modern rackets are light but they are fragile too. Depending on the racket material, stringing the racket at high tension may cause the racket frame to crack or even break. In my racket listing, I do include the maximum string tension for each of the racket that is safe. Of course, you can always tie it at a higher tension, but do it at your own risk!

Choose a badminton racket that could handle the string tension that you need, to prevent damage to the racket during the stringing process.
In the market, there are different strings available, some are thinner and some are thicker. A thin string will be less durable (e.g. 0.66mm circumference string) if tied at high tension, such as 27 lbs and above. However thin string will give you more power because it’s more bouncy compared to thicker string.
Another note that we want to point out is when a string tension is too high for you, it will normally cause vibration to your hand and arm, just like what an over-stiff racket will do to you, especially when you miss the sweet spot. Over the time, this will cause injury to your hand and arm. So be wise when choosing a suitable string tension for yourself.