A cocktail garnish is like a fancy decoration that goes on top of your drink. It's kind of like how you might add sprinkles to your ice cream to make it look pretty.
Sometimes, bartenders will put fruit like oranges or cherries on the edge of your glass. Other times, they might use flowers or herbs to make your drink smell or taste nice.
The garnish is meant to make your drink look and taste better, so you might want to take a small bite or sniff of it before you take a sip of your drink. But be careful not to eat too much of it, as it might not taste very good by itself.
So next time you see a fancy drink with something pretty on top, that's a cocktail garnish!