A mulch is a material such as peat moss, grass clippings or straw which is spread over the soil in a layer under plants to protect and improve the area.
Mulches serve many purposes.
They control weeds by preventing weed seeds from germinating and smothering small weeds in the area being mulched. They help keep soil moisture available to plants by preventing water from evaporating the soil surface. They prevent the soil from eroding or crusting over by providing a covering for the soil surface so that the rainwater trickles through the mulch rather than splashing directly on the soil surface and running off. They improve the appearance of the home by keeping out weeds and otherwise improving the appearance of the bare soil surface. They help keep fruit, such as tomatoes, up off the ground which results in cleaner fruit with less ground stain and decay.