A broody hen is a chicken that has decided to sit on and hatch a clutch of eggs. The eggs may be fertilized or unfertilized. The broody hen will sit on the eggs day and night, leaving only once daily to eat, drink and poop. If you try to remove her from her eggs, she may hiss at you and peck you.
Broodiness is driven by several factors: genetics, hormone, instinct and lighting conditions. Some breeds are more prone to going broody than others.
Buff Orpingtons, for example, are very broody hens.
And if a hen has a dark, comfortable, undisturbed spot where she can nest, she may roll a clutch of eggs (usually twelve to fourteen) to that spot and begin to brood. She will pluck the feathers from her own breast so that her moist, warm skin keeps the eggs warm, and the feathers insulate the nest. If anyone tries to disturb her, she will hiss, growl, shriek, puff up, and peck at you.
If You Want Her to Hatch Baby Chicks
If you have a rooster and you want your flock to reproduce naturally, you may be pleased if a hen goes broody. What you should do is remove her and her clutch to a separate area with food and water. This way, another hen won't force her off her clutch, leaving them to get chilled and die. You may need to make sure your broody hen eats and drinks enough by physically removing her from the nest and bringing her to her food and water dishes.
It takes 21 days for a fertilized egg to develop into a new baby chick and hatch, so keep an eye on the calendar.
If You Want to Prevent Her From Hatching Eggs or "Break Her Up"
If you don't want your hen to hatch eggs or don't have fertilized eggs, you will want to "break up" your broody hen.
This just means stopping the brooding behavior. It's important to do this as broodiness becomes "contagious," with other hens catching the idea and sitting on still more clutches of eggs. And being broody for too long is not the best for a hen's physical health.
To break up a broody hen, you can start by trying to disturb her when she is in the nest box. You may need to remove her to a separate place without access to the nest boxes, but with access to food and water. You may also try setting a clutch of ice cubes in the nest box so that when she returns to it, it's unpleasant.