It is a large polypore growing in Northern climates, preferrably on dying birch and beech trees. It has been used for making Amadou for centuries, even millenia. Recent clinical studies confirmed its many medicinal properties. You can use it to make a tea with anti-inflammatory and antibiotic properties that doesn't taste too bad. It is reputed to be anticarcinogenic, too. Please keep in mind, though, that I disclaim all liability if you mess up. Also, the fungus is protected in Germany. I have permission to harvest a smaller quantity for educational purposes. But please use common sense if you want to get some yourself. Taking one might not be a big problem, but please ask the local authorities first.
That said. I took one from a tree that broke off.What you are looking for is the piece underneath the hard grey skin on top. It is a small light brown layer without any layers that is called the Trama.
On this picture you can see the topmost and relatively small layer. On this tree I left a piece of the trama on the tree in order for it to regrow. In most cases you will end up with a leathery piece about as big as your hand and as thick as 3-10 mm. Whack it gently with a clean and smooth baton or the handle of your knife. By working it in either direction with your hands then you can stretch it, sometimes to as much as triple the size. The material can be used as is as a material for flint and steel fireststarting techniques or processed. You can also use it like leather. In Hungary and Romania, there are still living traditions of making hats and pouches from the material. Mine will become a pouch to be worn around the neck.
The material being antiseptic and very soft, it can also be used as a plaster for tending wounds. The fungus is rich with Polysaccharides and Triterpenes giving it its properties in wound healing.
That said, it is also a very satisfying feeling touching this awesome material. Maybe I will just keep it to touch it from time to time....;-)