Frites are french fries and  friteries are the places that fry them.  Frites are a national specialty, friteries, at least in Brussels are an endangered species.  Real estate is too valuable and to scarce to support a simple friterie.  Fortunately Brussel's best friterie has survived - Maison Antoine.

Frites are twice fried - first blanched at a low temperature and then, when ordered, finish fried to a crisp golden brown, one order at a time.  Traditionally they topped with a glob of mayonaise, which works fine, but other sauces are usually available.