An appetizer is part of a meal that's served before the main course. You might serve your dinner guests an appetizer of crab-stuffed mushrooms when they first arrive for dinner.
Usually, an appetizer is a small serving of food — just a few bites — meant to be eaten before an entree, and often shared by several people. You can also call an appetizer an hors d'oeuvre. An appetizer is meant to stimulate your appetite, making you extra hungry for your meal. This is where the word comes from, literally meaning "something to whet the appetite" or "something to appetize."