If you purchase something from a shop, you can describe it as store-bought. If you don't have time to cook for your friend's potluck, you can stop and pick up a store-bought lasagna on the way.
This North American adjective was coined in the early 20th century, right around the time the first grocery stores opened in the U.S. In Britain, most people say shop-bought instead. Store-bought is a good word to use for distinguishing food that's sold in stores from homemade: "I made my grandfather's award-winning brownies for the bake sale, but I noticed that everyone else brought store-bought cookies."