A fly tower is a part of a theatre above the stage where flat scenery in the form of gauzes, cloths and flats are stored and 'flown in' when needed. Stage lighting, rigged on lighting bars, may also be flown.
The scenery is fixed to metal bars which are then themselves supported by metal or natural ropes up through the tower to pulleys and from there down to one side where the flyman stands. They can be moved either by hand or via electric motor. Because of their weight they are usually counterweighted.
Historically, the flyman was an ex-naval rating, with experience with rope from working on sailing ships, and would communicate with the stage floor by means of coded whistles. For this reason actors and other stage crew would never whistle on stage in case a bar landed on them unexpectedly.