Open-faced is a term usually used to refer to some kind of sandwich, be it based on bread, bagel, croissant, or tortilla.
If it’s a sandwich, just one piece of bread is used. The sandwich fillings are put on that, and in the absence of a top piece of bread, the fillings become instead a topping, as it were.
With Open-faced sandwiches, the presentation of the content of the sandwich becomes more important because it is plainly visible. Often chopped herbs or other greenery are used as a garnish.
Open-faced sandwiches can be served hot or cold.
Canapés are teeny open-faced sandwiches in a way.
Sometimes hamburgers served with just a bottom bun are called “open-faced.”
Open-faced pies in England are called open-plate tarts.
The Danish call their open-faced Sandwiches “Smørrebrød.”