Green manures are particular plants which are grown to improve the structure and/or the fertility of the soil. They break up heavy soils and improve drainage. They can help light soils hold more water. Different green manures have roots of differing depths. These can mine for different minerals in the soil, bring them to the surface and keep them there. Green manures of the Legume family increase Nitrogen in the soil which will benefit the following crop. Some, such as Alfalfa, root down very deep and bring up minerals to the top layer of the soil. The green tops, when dug in, add humus and nutrients.
The gardener should turn the plants into the soil at the appropriate time for each variety. Some Green Manures can be clipped during their season and these clippings can be left on the soil as a mulch and then dug in eventually to improve the tilth still further.
If used in a rotation system, check the family of plants being used. Red Clover, for example, being a Legume would fit into a rotation in place of beans, and would be followed by a leafy vegetable such as spinach or lettuce.