Garnish can be anything added to food or drink before serving with the primary aim of providing decoration. Of course, flavour should not be overlooked; serving a fried egg topped with sliced strawberry, for example, is only for the most daring or eccentric of chefs.
Some time in our past after the development of agriculture, a steadier food supply became available for many people, and food stopped being a simple matter of nutrition. It developed into a pastime, sometimes a status symbol, or even an art form. That's where garnish comes into its own - the final flourish of the culinary artist's brush.
Common Examples of Garnish
Parsley - this is perhaps the most common and unimaginative form. It can be served either as a sprig atop any number of foodstuffs, or as a sprinkling of dried and crushed leaves.
Mixed salad leaves - with many meals, a side salad of various kinds of lettuce, rocket or other greens is served on the plate.
Dill - This is often used for fish - as well as being decorative, it imparts a flavour which complements fish well.
Tomato - Sometimes simple slices, sometimes cut more imaginatively into the shape of a 'flower' or 'basket'.
Fruit - Often with desserts, slices of fruit such as strawberry or kiwi are added on top.
Chocolate shavings - Also normally only for sweet courses.
Olives - Considered by many to be mandatory in a martini
Sauces - Often used for decoration as much as flavour, especially when drizzled artistically over or around the food.
Many more ambitious forms can be found; carrots or radishes carved as flowers or stars, or even an apple carved in the shape of a swan. This is beyond the scope of most of us, however.
People vary widely in their opinions about garnish. Some people feel that 'the first taste is with the eye', thus presentation and decoration are quite important. Others see it as a distraction from the substance of the meal - 'pointless fancy stuff'.
It also plays an important role in etiquette - many people frown upon eating it, and feel it should be left on the plate to indicate you are finished and that what you were served was quite filling. Others feel that it is good manners to clear the plate entirely.
Whatever your opinion, the practice has been around for a long time, and isn't likely to disappear soon. If you don't like it, perhaps you should just resign yourself with a smile!