Broody is when a chicken decides to sit on a clutch of eggs for several weeks, so that they'll hatch. Fertile eggs don't start developing until they get warmed up by the chicken's body or an incubator, to a higher temperature. They keep developing as long as they stay at that warmer temperature. Until then, they're dormant, like a seed that waits for warmth and water to start germinating. In the beginning, the chicken can come and go off the nest, laying an egg each day. When she has enough eggs, she'll settle down and stay on them for several weeks, until they hatch.
Now, some chickens will go broody on one egg or a golf ball. Some will try to brood more eggs than their body can cover. Some never go broody. If she starts to go broody on only a couple of eggs, you could quick put a few more fertile eggs under her. If you have a few in the house, you could give them back to her.
