They blow out their feathers (some drastically all over, some sparsely all over, some in patches) in order to grow new ones in. In more severe molts they look like porcupines with patches of pinfeathers filling in the bare spots. The sensation of pinfeathers pushing through the skin seems to be very irritating so chickens do not like being handled or touched during molt, and may get cranky with one another as well.
You will see decreased or a complete stop in laying.
Increasing protein can help with new feather growth but other than that, best to just minimize handling as much as possible and let nature do what nature does.