Rod,

Oldguy is correct. Essentially, it is tinder that ignites fairly easily with a firesteel spark. The fatwood block in the photo will last a long time and is capable of starting many fires. All you do is shave off tiny pieces with your knife (RMKs work best ), make a pile about the size of a quarter, and scrap a spark into it.

Fatwood is man's best friend that doesn't have a tail.

Link.