The description/name of this alert is a little vague, but in the basis sense, a wireless-bridge would be an access point that is wireless connected directly to another access point. If you are seeing this event alert, it should specify the access points in question. The first thing to determine is if any of the APs are 'yours'. If none of the APs are yours, then this alert can be ignored. If any of the APs are yours, you need to confirm that their bridge connection is expected behavior.