This is the racket balance point, measured in millimetres from the bottom of the racket handle.
The larger the number (head-heavy balance), the more power and stability you'll get from the racket and the heavier it will feel (even if it's not a heavy racket). However, you'll have to give up some manoeuvrability. The smaller the number (grip biased or handle heavy), the easier the racket will be to handle, but the less power and stability you'll get.