Green manures are plants that are specifically grown to improve the quality of soil. These plants take up nutrients, that would otherwise be washed away, and return them to the soil when they are cut down. They are allowed to grow for a few weeks and then either cut down and dug into the soil or cut down and left as a mulch.
Green manures can also protect soil from erosion and act as weed suppressants. Some, such as clover, actually take up nitrogen from the air which increases soil fertility.
There are quite a lot of plants that can be used as green manures, some of which are hardy and can be sown in the autumn for over wintering.
Clover and ryegrass may regrow and is therefore best cut and covered with a mulch to kill them.