A creance is a long light cord used to tether a flying hawk or falcon during training in falconry. They are used both when the bird is young, and when the bird has been taken out of the aviary for a moult or other reasons.
The creance itself is thin, designed to be of as little inconvenience to the bird as possible. It's threaded through the swivel and tied with a falconer's knot. The creance typically extends to around 25 metres (82 feet), at which point the bird should be ready to fly free (without the use of the creance).