Bracket fungus growing on a Beech tree - Possibly Fomes Fomentarius (Tinder Fungus).
Bracket fungus generally means that the tree is coming to the end of it's life. It could take decades for the tree to weakened to the point of collapse though and when the tree dies the bracket fungus will carry on feeding for decades more. It's just another natural cycle.
Dried Bracket Fungus can be used to make a good tinder for fire starting called Amadou. The drier it is the better. Inside the fungus is a fluffy middle layer. Separate this from the tough cap and the tubey bit on the base. This is harder than it sounds! The gray part on the top of the fungus is rock hard. Remove the fungus from the tree, being careful not to stab yourself, turn it upside down and remove as much of the underside layer as you can, you should then be able to scrape out or scoop out the middle bit that you're after! Make sure your knife is sharp and you are mindful to cut away from you at all times.
Using a flint and steel, drop sparks into the pith you have just collected until it holds one and starts to smolder. You can then build up your fire from there or carry the smoldering fungus with you, in a fire proof container, to your next stop.