If you on Las Rambla then it is THE place to visit. It's a very colorful market selling almost everything you can think of food. They sell ready food and also fresh produce which can be cooked at home. The shops in the end are little cheaper than those in the front. If you are a serious buyer then compare the price before you buy anything. More than 90% crowd are tourists so its going to be little costlier than any supermarket. We ended up buying a fruit platter which we enjoyed thoroughly. It's always very crowded and it's closed on Sundays.
Worth visiting...