That's pretty simple. If you have forests, you have trees, and trees are wood. You also have undergrowth, which can be flammable. After a long hot summer or other dry period, it's like a bonfire waiting to be lit. Some arseholes start fires, either deliberately (major arseholes) or accidentally (minor arseholes). But even without arseholes, lightning can start a fire.
If you're anywhere near a forest fire, get away as fast as you can. Because they can spread really fast.
To put out a forest fire, you need a bit of advance preparation. If you look around forests in England, you'll see wide lanes where the trees are all cut down, these are called firebreaks. The idea is to make it a lot more difficult for the fire to spread, it will find it hard to cross a wide lane where there's no flammable material.
And then, mostly, you dump water on the fire, or something else that smothers it. But you do it mostly from an airplane, so you don't get burned.