It isn't slang. It's the name for a kind of food--specifically, a kind of waffle. Here's a picture of one:
A waffle is somewhat like a pancake. It has to be formed by putting batter in a waffle iron, which molds it into a grid shape. If you put butter or syrup on a flat pancake, it runs off. The squares on a waffle hold it, so you can put more butter or syrup or other toppings on top.
Waffles are popular for breakfast in the United States. They are usually topped with things like sweet syrups, fruit, perhaps whipped cream.
"Belgian waffles" are a style of waffle. They are a little lighter and they have bigger squares than ordinary waffles.
"Belgian" is the adjectival form of Belgium. Belgium is a country in Europe. The capital of Belgium is Brussels; the headquarters of the European Union are in Brussels so it is an important European government center. The people of Belgium are "Belgians," and things from Belgium are "Belgian." There is no language called "Belgian." Languages spoken in Belgium include French, Dutch, Flemish, and German.
Belgian waffles is a type of waffle covered with whipped cream and/or fruit, a breakfast food. The term is mostly used in North America, the Belgians have different names for different sorts of waffles.
Here's a sentence example:
"It was on a visit to my husband's relatives in Europe that I was given this Belgian waffle recipe."