This is the racket's balance point, measured in millimetres from the bottom of the racket (handle).
The higher the number (when the balance point is closer to the head), the more powerful and stable the racket will be, giving you the impression that it's heavier (even if it doesn't actually weigh more). However, it will be harder to handle. Conversely, the lower the number (balance point towards the handle), the easier the racket will be to handle, but it'll also be less powerful and less stable.