Hi Cape townbear, and welcome to cruise-chat.com!
Generally, the surcharge includes the cost of the food. You'll pay extra for alcohol and non-alcoholic beverages other than tap water, iced tea, hot tea, or non-specialty coffees. In some restaurants, there will be an extra charge above the surcharge for some items. For instance, LeBistro offers (at least the last time I was there) a surf-and-turf option that included lobster that came with extra cost. Even so, there are plenty of options within the restaurants that do not require payment above the surcharge.