Brooding is an instinctual behavior that occurs when a hen's brain tells her its time to hatch babies. She will sit on the nest all day and all night and lay on a clutch of eggs and in theory hatch them. She will usually only get down once a day to eat, drink and poo and then go right back in the nest. She will also pull out her tummy feather to get her warm body right on the eggs. She will be very upset when anyone or anything comes near and will make noises like a dinosaur.
If you give a broody hen fertile eggs she will make chicks for you. Chicks do not start to develop inside fertile eggs until they go under a broody or into an incubator and the conditions are right. But, chickens cant count so even if you do not have a rooster or even if your hen has no eggs if she goes broody she will sit in the nest anyway.
Some breeds are not prone to broodiness than others.