Hello! I am a newbie, but from what I understand its just like when birds bathe in a water puddle. They "clean" themselves in it. I have 7, 8 & 9 week old chicks and have only saw a few dust baths so far.
It is really funny to watch tho.
The other girlies come over and pick at the dirt and stuff and seem to be helping
For birds that take dust baths, the activity is important for feather maintenance. The dust soaks up excess moisture and oil and probably helps to remove tiny parasites that live on feathers. Its thought that dust also soothes irritated skin and discourages skin parasites that infest birds. Taking a dust bath may even help birds cool down on hot days.