A garnish is a decoration or embellishment, often used with food. It is also the verb that means to do the decorating: you can garnish a baked fish with a garnish of lemon slices and parsley.
Long ago, in 14th-century France, the word garnir meant, in part, "to warn or defend," a meaning that survived in one of the meanings of garnish, "to seize wages to pay a debt." But word meanings often branch out, and "to warn" came to mean "to arm oneself," and later "to fit out or equip." It came to mean "to embellish" in Middle English, and, from the late 17th century on, the word was used commonly for the embellishment or decoration of food.