A mop sauce is simply a "marinade" or baste that you apply to the brisket during cooking. The name mop sauce came from the utensil used to apply the sauce as you seen in the picture.

 Moping your brisket during cooking will help impart flavor, keep the "crust" moist, help prevent burning, and help improve the carmelization of the meat.

That said, here’s the recipe

The Ultimate Brisket Mop Sauce

1 cup – Your Favor Lager or Ale
1 cup – Good Apply Cider
1/2 cup – Apple Cider Vinegar
1/2 cup – brewed coffee
1/4 cup – Grandpa Murray’s Texas Spice Rub
1/4 cup – Worcestershire Sauce
1/4 cup – brown sugar
1 Tablespoon – Hot Sauce
Salt and Pepper To Taste

Place all of the ingredients in a non-reactive bowl and whisk together. If you are going to use the sprayer method of mopping your brisket, use a fine strainer or cheesecloth to strain the spice rub from the liquid, and pour into your strainer. We highly recommend cleaning the strainer with boiling water for 20 seconds before and after using this mop sauce