A mop is a basting sauce. It adds flavor and keeps the surface of the meat moist. I make a jarful of apple cider vinegar mixed with a little olive oil, prepared mustard, worchestershire, soy sauce, lemon juice, wine, and whatever else in the cupboard looks interesting, but never anything with sugar. Sugar will eventually burn, so if you’re inclined to brush on any bottled, tomato-based sauce – always loaded with sugar – don’t until the last few minutes.
Mop sauce must be daubed, rather than brushed, onto the meat. You want to avoid wiping off the rub. Cooking shops sell miniature mops, or you can tie a clean rag onto a wooden spoon with baking twine. Or just use a brush, but don’t brush it. Mop every half-hour or so. More often lets the heat and smoke out of the pit.