They just sit in the nest and never get out to eat or drink much. Also, they are done laying eggs for awhile. They will lay a certain amount, then stop to just sit on them until they hatch.
I alway's Knew my hens were going broody because they would start losing/plucking their feather's on their breast. My girls would puff up like a turkey and scream their little heads off of if I went near the nest.