Q: What is molting?
A: Molting occurs once a year in mature birds, normally in late summer or autumn. The birds will lose their feathers and regrow new ones. It's a way of refreshing their plumage for the winter so they'll stay warm. Young chickens go through a few molts in their first year, however, as they're growing in their adult plumage.
Birds can go through either a hard molt or a soft molt. A hard molt means all the feathers are lost almost at once so molting is over relatively quickly. A soft molt means the feathers are lost and regrown gradually. Sometimes you may hardly notice a soft molt, except as a reduction in laying. Molting takes a few weeks or more, depending on the "hardness" of the molt--it can seem like a hardship when you don't have delicious fresh eggs!
What type of molt your birds have is usually a matter of genetics. Commercial entities tend to breed for a hard molt, because birds lay fewer (or no) eggs while they're molting, since feather growth uses up so many of your hen's resources. Natural molting, hard or short, is brought on by shorter days, but a molt can be caused by other triggers. For instance, factory farms will often starve a bird for several days to induce them to molt all at once so it's easier to manage. Can you imagine the type of person you'd have to be to think it's okay to starve chickens if it's done in the name of the bottom line?
You can imagine how My Pet Chicken feels about this abuse!