Red Mites are horrible little mites that live in the tiny cracks in the wood of your coop (or in felt roofs).
At night they come out and feast on the blood of your chickens. Very soon the population will increase massively and as more blood is taken from your chickens, they stop laying, become depressed, and can eventually die.
The test for red mite is usually running your hands under the perches in your coop in a morning - if you have tiny brown or red smears then that's the your chicken's blood from the mites' feasting.
You can treat it with pesticide but they are notoriously hard to eradicate this way.
Jeyes Fluid will kill them, but again it's hard to completely treat the whole coop.
DE earth is made up of very fine crystals which cut the outer shell of the mite and kills them mechanically rather than chemically, so mites cannot become immune to it. They literally dry out and die.
If you have DE earth, you just sprinkle it everywhere in the nest boxes, sleeping areas, even the coop floor. It's safe for hens to eat so don't worry about it if they get some on themselves (it's a good idea to add some to their dust bathing area). Some care has to be taken not to breathe in the dust as it can cause respiratory problems though.
An easy way to treat the inside of your coop safely is to use a spray solution like Poultry Shield - it's cheap and easy to use and very effective.