BSSID is the Mac id of a wireless access point. in case you use multiple wireless networks overlapping, BSSID [Basic Service Set Identifier] will come handy to identify to which access point your device connected to. This small article explains how to find wifi BSSID on Mac.
How to find WiFi BSSID on Mac
Hold the option key and click on the WiFi menu on the menubar. Then you can see the BSSID address.