The stuff you see in the photo is dryer vent lint. And in this case the dryer vent was completely stuffed. Dryer vent fires are more common events than one might think. But if you look at it not necessarily unexpected. When you dry your clothes, you are using flame and very hot air, in the case of a gas dryer. Dryer vents can get full of a very fine lint, which is extremely flammable, that combination can produce “dryer vent fires”.