The Falconers Knot is used to secure a bird of prey to a perch. Actually when I say secure, the bird does have a fair amount of movement until it gets to the end of its tether. The whole idea of the falconers knot is that it can be tied with one hand, as the bird of prey usually occupies the other hand, making it difficult to use two hands to tie the knot.
The falconers knot is a really quick and simple knot to tie. To see how simple just take a look at this short video: