To enable geofenced WLANs, navigate to Network -> WLANs to create a new WLAN. Scroll down to find the Geofence section where you can set the allowed RSSI range in dBm for clients devices. This is usually done to try and make sure a user is within your facility or coverage area before offering them service.
Note: This proximity based access is only for the initial association. If a client is already associated to the network, they will not be dissociated if their RSSI value falls below the configured threshold.