A mulch is a soil covering used to keep weed growth to a minimum, to conserve soil moisture and help insulate plant roots from extremes of cold and hot weather.
Mulching with black plastic for instance is good for marrows and can be used to produce crops of potatoes without having to earth them up.
Most mulches, however, are made from organic material - such as those used to improve the soil structure (compost, well-rotted manure, leafmould etc.). To be totally effective they need to be around 5cm (2in) thick, but mustn't make direct contact with the plants.
More on mulches and mulching