It is Diatomaceous Earth.
From Earth Works Health - "Mined from the purest of deposits from prehistoric freshwater lakebeds.
Food Grade Diatomaceous Earth consists of finely milled silica shells of
ancient microscopic freshwater plants called diatoms. Of 600 deposits in
the US- only 4 rate in purity by FDA standards to label as "Food Grade".
Examples of Food Grade Diatomaceous Earth Uses for Pets & People:
Flea control, human health benefits, organic pest control, kill bed bugs,
pet food health supplement, cat litter odor control & whiter teeth!"
Most people here, me included, use it as a dust in our coop to help prevent bugs, mite, lice, etc from getting to the birds. Many also dust their birds with it. The chickens can also eat it which may help / prevent worms.
If you get some you want to get the food grade DE, there is a similar product used for swimming pools, but it is not guaranteed safe for consumption.
I am sure more people will chime in, many use it, I just wanted to give a brief description. I recommend it! We have already seen great results in our dog run and calf barn with the reduction of the amount of flies and smell of the barn.