Fatwood is that part of a tree that collected pitch. In conifers such as pine, fir and spruce, it's typically from areas of the tree where limbs grow out of the main trunk (calles a pitch knot) also scars, lightning strike areas from partially burned trees that recovered.
Fatwood is typically heavy nd dense. If you were to try to saw it it might leave melted resin on your saw blade. Natural fuel; good stuff