Community supported agriculture is exactly what it sounds like - local people supporting local farmers. At the start of a season, you sign up for a share and agree to pay the farmer a fee. In return the farmer agrees to supply you with fresh produce for a set number of weeks during the growing season.
A consumer walking into a typical grocery store has no way of knowing where or how the tomato in the produce section was grown, other than perhaps a general location as indicated on a sticker, "Grown in California".
By buying into a CSA plan, you know exactly who, where and how that tomato was produced. You become much more in tune with the growing season and much more accustomed to the taste of fresh seasonal products. Often CSA members are welcome and encouraged to visit the farm to take a look and even help out.