A friterie is to a Belgian is what McDonalds is to an American! Scattered around the country you often come across these friteries which sell as they main item "frites". They're also called "pommes frites" since they are made of potatoes.
Just about everything that you order in a friterie is automatically thrown into a great big fryer. So your frites, your hamburgers, your chicken nuggets are deep fried. The frites are actually cooked twice.
With the frites, the main "saus" is mayonnaise! No kidding! This tastes nothing like Hellmann's or your typical mayo, but has a more unique, perhaps a bit sweeter flavor. If you're not into mayo, then you can have ketchup or one of many other sauces - saus americain, sambal, picante, pita, etc.
Usually accompanying this meal is of course a Belgian beer. Stella Artois and Jupiler are the mainstream lagers.
Sure, this place sounds like a recipe for a coronary but in moderation it will not kill you.