What do they mean by urbanization = fuel buildup for wildfires?

Question

What do they mean by fuel? Is it anything that promotes the ignition and anything that allows a fire to continue burning? (Dry old wood/grass, surface oil, any ignition sources left by humans etc?)?

Answer 1

Every time there's a major house-destroying wildfire in California, high-density housing receives part of the blame. Neighborhood after neighborhood of houses packed tightly together.

That's my guess.