Mulching, with respect to gardening, is the act of placing a protective layer of some material over the soil to retain moisture. There are a lot of materials which can be used as mulch, including organic residue (grass clippings, leaves), and compost.
Some plants respond well to mulching, while others do not.
But what about raspberries? Should raspberries be mulched?
It really depends on the type of soil they are planted in. If the soil is already wet and moist, then there is really no reason to mulch your raspberry plants. However, dry soil can benefit greatly from a layer of mulch.