Birdnesting means that the children live in the same house every day and that the parents take turns living there with the children for a number of days.
This gives a lot of peace and quiet for the children. But it requires a lot of adaptability from the parents. The parents also loose some of their privacy. For the parents it is most pleasant if during the other days they van stay in a pleasant place (their own home), where they do have privacy and can see a new partner, for example.
Bird nesting can also be used as a temporary solution if the marital home has not yet been sold or if the parents have not yet found other living space. They can then take turns staying elsewhere, to give the other parent some space.