The loading bridge (aka loading gallery) is a platform located just below grid level directly above the lockrail and/or pinrail. This location allows a loader to add/remove counterweights as needed to counterbalance the load on the batten.
For more architectural and fly system specific terms, see:
http://www.northeaststage.com/store/Counterweight-Stage-Rigging-p-1-c-29.html
For more architectural and fly system specific terms, see:
http://www.northeaststage.com/store/Counterweight-Stage-Rigging-p-1-c-29.html