A mop sauce is comprised primarily of oil and different types of flavorings. It is applied to the meat at intervals of anywhere from 30 min. to an hour. Many believe that mopping a meat will not only add additional flavoring to the meat but more importantly to keep the meat moist during the long cooking process. This type of mopping should not be confused with the application of a BBQ sauce at the end of the cook.
As with many things dealing with BBQ, there are a couple schools of thought when it comes to Mopping. Many BBQer's were taught from the beginning that you should always mop. However over the years many people have found that mopping not only doesn't affect the moisture level of the meats but extends the cooking time due to the opening and closing of the smoker. Each time you open the lid of the smoker it loses heat and takes anywhere from 10-30 minutes to get back to your target temperature, resulting in a longer cook.
Still others have found a middle ground and mop only for the additional flavoring and only in the last hour of the cook. It basically boils down to individual preference.
I myself sometimes mop for flavor and other times don't. For those of you that wish to mop I have included recipes for mop sauces and indicate that you should not mop more then every hour unless you don't mind longer cook times.