Mulching is adding a layer of material on top of the soil to stop the soil from drying out. It can also have a number of other benefits, including warming up the soil, but these depend on what material is used. Some materials, black polythene, biodegradable paper, or newspaper for example, will also act as a weed control because they cut out the light from growing weeds.
The papers can be dug into the soil at the end of the season. Compost and manure mulches will eventually he drawn into the soil surface by worms and so continue to feed the soil and improve the structure but weeds, like the vegetables, will flourish. They are, however, more easily removed from a mulch. White plastic reflects on to plants and aids growing and ripening fruit, but does not suppress weeds.
Lay plastic mulches when planting out, either between rows of plants or cut crosses in the plastic and plant through them. The easiest way to water plants grown in this way is to lay a length of flat seep hose beneath the plastic and leave it there. Less water will be necessary as the plastic stops evaporation. Any sheet material and especially plastic needs to be held down securely or it can easily blow away. Place it on the soil and with a trowel make 10cm/4in slits in the soil along the edges. Feed the edges of the sheet into the slits and push the soil back over the top to hold the edges in place.
If you are using loose material, allow seedlings to come through before you spread it. Use enough material to make a layer at least 5cm/2in in depth. This can be put down in advance when planting out seedlings and simply be pushed to one side while planting, then moved back into place around the plants afterwards.