Companion planting means growing certain plants together with the hope and intent that one will benefit another or that both will benefit each other. Companion planting is a bit of art and science; not all of the claims for companion planting have been scientifically proven, but anecdotal evidence and collected garden wisdom seem to support keeping some plants close and others at arm's length.
One of the most helpful influences one plant may have for another is the ability to repel pests and or attract beneficial insects. Other plants--mostly legumes--aid other plants by helping to enrich the soil. If you decide to give companion planting a try, keep in mind that companion plants do not need to be bosom buddies. Vegetable garden companions will still have a positive impact even if they are many feet away or several planting beds apart.