Hi chicken clown,
Welcome back to the world of chickens. DE stands for Diatomaceous earth which is a fine, light, powdered substance formed from the remains of deceased, fossilized, single cell algae, or diatoms. DE has a typical composition of more than eighty percent biogenic silica. The remainder is composed of clay minerals, notably alumina, and iron oxide.
You can purchase food grade DE at almost any local Feed Store. You may even find it at a local grocery, but for the purpose used for chickens, it doesn't cost much to purchase a 25 or 50 pound bag and it you stay on top of your chickens, it will get used.
It can be scattered in their food, it can be dumped in with a pile of dirt and sand for them to bathe in and it can also be applied directly on the chicken. It is used for treating biting lice, bugs, worms, fleas, etc. When using, make sure you are in a well ventilated area as it is very light and creates quite a dust cloud that you don't want to inhale. It makes sense not to create a dust cloud for the chickens to inhale as well. It's wise to purchase an inexpensive paper breathing mask to keep around for when you are dusting the chickens. We do it outdoors so the dust can fly. Scatter some in the coop, in the nesting boxes and just about anywhere a chicken might sit for a while.
We try to always dust broody hens right before they hatch out their clutch, because red mites can kill little chicks quickly.
It can be used for other livestock as well. Whether dusting some on their food, or applying directly. Some say that once it gets wet it loses it's use, but I have also read the sharp edges of the silica are not destroyed when wet. We use it because it's natural.
Enjoy your chickens.
