A mulch is a layer of material applied to the surface of the soil to conserve moisture, suppress weed growth, improve fertility and soil health or just applied cosmetically to make an area look good. During maintenance TUF mulches around new trees both to conserve moisture and suppress weeds that may compete with the trees for water or nutrients.
There are inorganic mulches such as gravel, felt or plastic coverings and organic mulches such as manure, bark or leaf-mould. Organic is best because it will be absorbed over time disappearing as the trees mature; TUF uses bark chips.
This is how its done: