To muddle is to combine ingredients, usually in the bottom of a mixing glass, by pressing them with a muddler before adding the majority of the liquid ingredients.
A muddler is a long pestle shaped often shaped like a baseball bat that is commonly made of wood, but modern designs can be found in stainless steel or plastic with teeth on the bottom. One end of this essential bar tool is large and rounded and is used to mash the ingredients. While the other end is skinnier and flat and is used to mix ingredients.
When stocking a bar, it is ideal to have both a traditional wood muddler and a plastic toothed muddler. Use the wood muddler for herbs, such as mint, for Mojitos and Mint Juleps. Use the newer, toothed designs like Shawn Soole's Viva Stick, for fruits and other thick ingredients.