Muddling is a technique to bring out flavors in fruit and herbs, but without destroying it, as a blender does.
Place the fruit or herbs in the bottom of a glass. Some drinks also will muddle a sugar cube with the other ingredients. With a bar tool called a muddler (or some other kitchen item with a small flat base you can use to muddle), repeatedly mush the items into the bottom of the glass.
If you need to purchase a muddler, you can choose either a wooden muddler or a plastic style. Some also have tiny comb-like teeth on the bottom to assist with the muddling.