Cover crops, also known as green manures, are used by gardeners to add organic matter, protect the soil from erosion, suppress weeds, and add nutrients to the soil. Gardeners typically plant legume cover crops help to fix nitrogen in the soil, while non-legumes add biomass, known as organic matter, to the soil. Cover crops can be used in a variety of ways. Most commonly, they are planted following an edible crop or in fallow areas, grown for a period, and then killed and worked into the soil. Cover crops can also be intercropped by planting between rows or around desired plants to provide a living mulch for weed suppression.
One cover crop group of note is those in the brassica family, such as radishes, mustards, and rapeseed. Research has shown brassicas to exude compounds that essentially are a biofumigant and have been shown to suppress weeds, insects, nematodes, and plant pathogens. For more information, check out this great factsheet HGIC 1252, Cover Crops!