Mop is a liquid put on the meat during the BBQ process.
A typical BBQ mop is a thin liquid with vinegar or fruit juice as its base. Ingredients added to the base include beer, rum, bourbon, spices, oil, and the rubs used on the meat.
The mop is applied periodically throughout the cook to the meat by brushing, spraying, or even using a BBQ mop. This process is used to help keep the meat moist and add flavor.
The mop is usually warmed prior to applying so the temperature of the meat does not drop. You can keep the mop in a pot on the cooker to keep warm.