A bit worrying when you see it in action for the first time, but try to remember its a natural process where a hen is preparing herself for motherhood. A broody refuses to leave her nest, becomes very protective of it to the point of violence, makes an agressive purring or shrieking noise when you try to move her, and will probably pull some feathers off her chest and underside - this lets her body get closer to the eggs and keep them at a more constant temparature. A hen will stay broody for as long as two months and won't lay during this time. She'll stop when she gets bored, or when she hears the pitter patter of tiny beaks...