Mulching refers to the process of returning the grass back to the lawn, so to speak. As you mow, bits and clumps of blades of grass fly up and bunch together. Mowers with mulching functions gently return the grass trimmings to the ground after you have cut them. It is usually either in extremely fine trimmings, or in clumps. It is a way to nurture your yard and saves you from having to collect yard waste.