A girl chicken is called a hen.
A boy chicken is called a rooster.
A hen lays an egg every day, but unless there is a rooster the egg will not be fertile and will never hatch into a baby.
The rooster jumps on the hen and breeds her and the egg gets fertilized inside the hen before she lays it. Chickens are not like fish or frogs where the egg is fertilized outside, with chickens the egg is fertilized inside the hen.
It’s important to keep an eye on the rooster and make sure he’s not too aggressive and hurting the hens. A polite rooster will do a little mating dance to trigger the breeding.
If there is a rooster with the hen the eggs she lays will be fertile, and if she sits on the fertile eggs for 21 days they will hatch into baby chickens.