This is how much signal your cable modem is receiving. Most cable modems like to have between 15db and -15db.

If you have more than 15db, it can cause interference and corrupted data, which will hurt your performance.

If you have less than -15db, you have a very weak signal. Make sure your splitters are configured properly for cable modems, or if you are on a loop system, consider running a new line for the cable modem or moving the cable modem to a better location.

(Thanks to soganta for a correction.)