Grapes are said to have complex carbs because they do not contain any refined sugars. Simple carbs are the ones that are found in most junk foods which can contribute to weight gain among other health problems. One of the reasons that fruits and vegetables are so good for us to eat, especially those who are on diets, is because of their complex carbs.
Foods that are considered to be complex carbs have three or more linked sugars whereas simple carbs have only one or two linked sugars. Anyone who wants to eat as healthy as possible will want to stay away from foods that have simple carbs because they require exercise to work off whereas foods with complex carbs do not.