A muddler is a bartender's tool, used like a pestle to mash—or muddle—fruits, herbs, and/or spices in the bottom of a glass to release their flavor.
Cocktails that require the use of a muddler include:
Mojito made with light rum
Caipirinha made with cachaça
Caipiroska made with vodka
Mint julep made with Bourbon whiskey
Old Fashioned made with whiskey or brandy