Mopping, basting, spritzing - they all achieve the same affect so its really more about preference and convenience. While a bit different than brining, injecting, or marinating, it's all about keeping the meat moist while enhancing flavor and is often used in conjuction with the above methods.
Mopping is just that - you use a small mop modeled after old world cotton kitchen mops than drench the meat with a single dip. Mop sauce is thin and runny and often the same mixture as used in the tray of a water smoker.
If you are cooking about about 265 degrees, don't add sugar to your mop sauce as it will begin to burn. Mop sauce should be very similar to what you'd use in a marinade, injection, or brine.
Another easier and a bit cleaner technique is to combine your mop ingredients and put them in a generic spray bottle and simply spritz the meat every 30 to 60 minutes and you can even use a turkey baster.
A super-simple mop sauce that works great:
1 part apple cider vinegar (Bragg's Organic Raw Unfiltered)
1 part favorite beer
Feel free to add wine, fruit juice, Worcestershire sauce, spices, anything that pleases your palette!
Q-Tip: If you marinated or brined your meat, boil the leftover mixture to briefly to kill the bacteria and use that as your mop sauce.
You can also find mop sauces in grocery stores alongside BBQ sauces.