The word is from the professional kitchen and means the visual, edible enhancement of a dish just prior to serving. Typically this means scattering over a chopped herb, but also describes a number of more complex formulae to finish dishes that include turned root vegetables, fried garnishes and so forth (see Duxelles, Flamande, etc.). The word garnish has rather gone out of fashion in food writing, being considered vulgar by some, though garnishing a dish is something most of us do just before serving, whatever we say to the contrary.