Brush has two meanings: 1) a tool for painting or scrubbing 2) a collection of small bushes and plants, spread evenly so that they seem like one big mass - brush is usually very dry So, a bush in an individual plant, but brush is a collection of them. Underbrush is brush that is under other trees and bigger bushes. During dry seasons, this brush itself is very dry and burns easily, and can catch the bigger bushes and trees on fire. So, to help prevent fires, fire crews clear brush (underbrush) from under trees.