Fixed-price or price fixing?
Prix-fisse, menu turistico, prezzo fisso, Gedeck—a fixed-price or set tourist menu may not be the best meal you could have at that restaurant, but it's certainly often the best budget option
Some people will tell you that a "fixed-price menu" is more expensive with more options than a stripped-down "tourist menu." Whatever the restaurant calls them, meals at a set price are always cheaper (up to 30%) than ordering the same dishes à la carte.
The tradeoff? Your options are usually far more limited than if you ordered from the main menu. Shop around. Is your only choice four different pasta shapes in tomato sauce, or are there more inventive dishes available? Is beer or wine included, and how much—a glass or a half-liter? Is dessert or coffee included?
However, as sampling the food is every bit as important as the museums and monuments you visit when traveling, I'd say splurge on what hits your fancy or seems to be the local specialty, and leave the spaghetti and chicken cutlet until you get home.
Another option, typically offered at nicer restaurants, is the tasting menu, a multi-course sampling of some of the restaurant's specialties (or perhaps chef's specials not on the regular menu). Through usually far pricier than a tourist menu, these can be an excellent value, especially as they often cost at least slightly less than were you to order an item off of each course a la carte.