A broody hen is a chicken that has become determined to hatch eggs.
A broody hen will leave the eggs for very short periods of time, to get food and water, but as the eggs develop she’ll leave less frequently.
Left unattended, a hen will stay broody for around 21 days, which is the time it takes to hatch a clutch of fertile eggs. After 21 days the behavior should stop at which point she will raise her young.