A trap nest is a nest box that has a door which automatically closes when the hen enters it to lay an egg. There are a number of different designs for the automatic doors that involve different (sometimes quite ingenious) trigger systems, usually mechanical.
Trapnests have a couple of uses. For one, it lets you know which hens are laying eggs. This lets you determine accurate which hens are your best layers. (There are other approaches that work, too, but they are a little more subjective).
Second, it lets you know which hen laid which egg. This can be useful in breeding programs. For example, suppose you have a flock of hens that you are pen mating with one rooster. If you have trap nests, not only will you know the sire that fertilized the eggs, but you’ll also know the hen that it came from.
We’re not currently using trap nests with our flocks but have considered it and may in the future on a limited scale.
The main limitation or downside with a trap nest is that you have to check them frequently to let the hen back out after she’s finished laying.