Barrage balloons were large balloons tethered to the ground with cables. These cables were designed to damage any planes attempting a low-flying attack. They were used in the United Kingdom during World War II to defend against bombing by the Luftwaffe.
Rose Tyler tried to use the cable of a barrage balloon to reach a small child who was stood on a rooftop. The balloon drifted into the open and Rose narrowly avoided the Luftwaffe aeroplanes flying around her. She was saved from plummeting to her death by Jack Harkness, who used a tractor beam to stop her fall. (TV: The Empty Child)