For many, deep litter means one leaves the bedding in place and merely adds to it as it decomposes or as needed for moisture absorption. It is a way to provide a place for the growth of beneficial microorganisms in the coop environment that can take some time to establish, but once they have a good growth, can inhibit the growth of harmful bacteria and fungus/molds.
This can take a little managing to create the right environment, for if it is too wet it will grow the wrong kind, too dry it won't grow the right kind, too ammonia it will create harmful fumes and kill the right kinds of organism....well...you get the picture. Once you get the knack of it, you can pretty much tell by sight, smell and feel of your bedding when and how to make the adjustments it may need to keep it working right.
Good deep litter is usually a golden, medium brown in color, can be picked up easily in the hand without clumping if you squeeze it...should be sort of mealy and should smell like good composted material~sort of earthy. Good ventilation is needed to use deep litter and I like to aerate the bedding itself by using "chicken power"...I just toss a handful of BOSS in the bedding during the wetter months of fall and winter and let the chickens toss the bedding around and break up any spots with a high moisture content.
I clean mine out twice a year but some never clean out their bedding for years...they merely take out some for gardening purposes and then let it build back up into the proper mix again with time. I don't use DE or any other additive to mine to "dry it out" as the chickens and the ventilation do this just fine. Some people say their DL is too dry but I'm thinking they have too much bedding to too little chickens or they have poop boards and remove all the good moisture that should be available in the fecal matter.
Some will claim you can't get DL to decompose unless you have a soil floor to your coop but just about anything will decompose if it has the right conditions and it doesn't require a soil floor to make those conditions right..composting can happen on many types of surfaces. I've been doing it on old oak flooring for the past 5 years and it decomposes just fine and makes a great addition to my gardens.
The main reason I do it is to promote good health in my chickens, but it also keeps my coop floor warmer in the winter and keeps my coop smelling way better than it did when I used to rake out the droppings every other day. It doesn't promote mite, lice or worms as many would claim...those who use true deep litter rarely, if ever, complain of those things in their flocks.