Mulch is a layer of organic matter which you spread over the surface of the soil. It can come from a wide variety of sources, including but not limited to, bark or wood chippings, compost grass cuttings, seaweed, leaf mould, and part rotted animal manure.
The purpose of mulching is to improve water retention in the soil and to suppress the growth of weeds. It will also improve the structure of the underlying soil.
There are some issues with the use of various types of mulches and it is a good idea to do research on a particular material before you use it. For example if you use grass cuttings, the bacteria which break them down will compete with your plants for the nitrogen in the soil and may impair the plant growth . This problem can be reduced by adding extra nitrogen for example pelleted chicken manure. Using horse manure (popular in my area of Berkshire ) can add weeds to the ground as horses intestines do not seem to be very efficient at killing seeds. Also if it is used while it is too fresh it will be quite acidic and scorch your plants. You can reduce the risk of both these problems by stacking it in an unused area and leaving it to rot down for a year or so.
Overall mulching is a very beneficial process, but make sure that you research the material you are planning to use.