Most mops are as Dutch mentioned; a mix of apple juice and your favorite adult libation. I've seen people use JD, Cpt. Morgan rum, etc.
Apple juice can also be substituted with any flavor you like; pear juice, white grape juice, anything really.
To that you can add whatver spices you might like; cayenne, garlic powder, onion poweder, cajun seasoning. The only limit is your imagination.
Be careful when you add anything with a high sugar content because it can carmelize and burn as was already mentioned. If you are going to apply a mop to meat that already has a rub applied, be gentle and let it set for a bit to avoid washing the rub off.
Some use a spray bottle as they feel it removes less of the rub.