An hors d’oeuvre is an appetizer. At a fancy party, waiters might walk around with trays of bite-sized hors d’oeuvres.
Hors d’eourves are served in small portions, like most appetizers. This term originally meant something out of the ordinary before it came to mean something separate from the main course of a meal. Other types of food that are similar to hors d’eourves in size are the Chinese dim sum and Spanish tapas.