Molting

Edith molting

If you notice your chickens are losing lots of feathers, most likely they are molting. If they are healthy, and if it's around winter time. The chicken's first year is always a little awkward, so they may not molt, or just a bit, or like my hen Edith, a full body molt every year. During this time, they will either decrease laying eggs, or stop until they have completely recovered. You will also know when they are not laying because their comb will be pink and dry, instead of thick and red when they are laying.
(They are NOT molting if you see damaged feathers/scabs/skin/blood on their back or head; then they are being hen pecked).

after the molt

Make sure there aren't any drafts in the coop, since their loss of feathers won't keep them as warm. One of my hens would sleep in the nesting box sometimes. If your hen/s seem cold and shaky, then you may need to put a heater in the coop or if its just one or two, you can put them inside the house, in a draft free room for the night so they don't get sick.
Molting is a very interesting process to watch. My girls actually seem embarrassed, and don't want me to come anywhere near them! They also don't feel great either, so I just leave them be. Then around a month later, they will be as good as new (literally) with pretty soft feathers.