Muddlers are used to muddle (read juice,crush,grind and blend) fruits,leaves and other ingredients in drinks similar to a mortar and pestle except in a cocktail glass or shaker to prepare a drink (such as a Old Fashioned,Whisky Sour, Caipriana among others). They are an indispensable tool for a well stocked bar.
They come in various sizes (from single drinks disposables to industrial/commercial) and materials (wood,metal,glass and plastic) depending on a number of factors. I always like to have a number of them on hand as some work better than others for a particular drinks and sometimes you want to include a combination muddler/stir rod in a drink.
A few tips:
1. If you are using wooden muddlers you should coat them with food grade mineral oil as you would a wooden cutting board - to prevent anything else from soaking in.
2. When mixing a drink where you are going to muddle fruit it is usually best to add the sugar first then the fruit,leaves, etc., as the sugar makes a good abrasive to extract more flavor (plus being on the bottom it helps dissolve the sugar)
3. Rinse any muddler off quickly when you are finished using it for the night (or day) and wipe it dry - fruit juices are sticky,corrosive and spoil.You don't want to compromise your drinks next time around.