This is the grass which has been set aflame by either a molotov, a frag grenade, a flamethrower or an explosion of some sort.
The flaming grass will quickly spread if left unchecked, and will take to ferns, trees and more. Your best best when in a dense, foliage-filled area that's been set on fire is to simply run away, as there's just no way to counter the fire on your own.
