Broody hen is a term used to describe any hen, or bird that is hoarding eggs in a nest to sit on and hatch.
Most hens today do not have the instinct today to sit on and hatch(brood) eggs. It has been selectively bred out of the layer breeds to increase egg production. However, almost all bantams and exotic breeds are very broody, to the point of fighting you for their eggs!
When a hen goes broody she will lay her eggs in a secluded nesting place or box until she has a sizeable clutch(bunch) of eggs. This can range from three to as many as fifteen! When the hen decides she has enough eggs she will sit on them and stay there until they hatch. Hens at this time will also stop laying completely, which is why it is being bred out of laying hens. The hen will sit on the eggs, keeping them warm, and rotating them every so often, until they hatch. The hen only leaves the nest for a few minutes each day to eat, sip some water and use the restroom. She will do this for up to 35 days or until the chicks hatch. If nothing happens after the 35 days many hens take the hint that nothing is going to hatch and they abandon the nest.