Cutman's explanation is perfect, but just to expand a little bit on the "cover crop" concept: Buckwheat germinates and canopies very quickly, when planted at the right time. This can be to your benefit because it creates so much shade that other stuff you do not want to grow either doesn't germinate or quickly dies, if it does.
Say you have an old pasture or meadow area and you want to convert part of it into an actual food plot. You can mow it and/or disc somewhere in middle to late Spring, wait a couple of weeks for new growth, spray that with 2 quarts/acre of glyphosate (generic for Round-up) and then plant buckwheat the same day. Before the stuff you sprayed has any chance of coming back, or new stuff germinating, the BW will take over. It acts very much like an additional spraying of gly; one that lasts 6-8 weeks before producing a lot of seed. In southern locations you can double-crop by allowing it to produce that seed and then flailing the plot before cultipacking. In northern locations, you can wait a little longer before planting and time the termination of the buckwheat to coincide with the planting of your Fall plot, usually a winter rye-based mix, like Cutman said.
