Broody is when a hen feels the urge to hatch out chicks. She will sit on rocks, golf balls, or nothing and stay there trying to hatch them. While the hen does this she can often become very protective to the point where she screeches, bites, puffs up the feathers and is generally very unpleasant to deal with. She may also steal eggs from other hens. If other hens lay in her nest box, she will take those over too.
Broodies generally do not want to leave the nest to eat/drink or poop. You have to force them out at least once a day so they can do that. Depending on how strong the broody hormones are a hen can be in that state so long (especially if the eggs are infertile and will never hatch) that it threatens her life. If you don't have fertile eggs for her you could get some and slip them under her at night so she thinks they are hers.
New broodies may not follow thru as they are inexperienced- they may quit sitting on the nest after a few days and go back to being a normal chicken.