The act of one male rubbing another male in any way that satisfies him. The tub-rub, as you might have guessed, must be conducted in or around a tub. It is not exclusive to porcelain bathtubs but can also be conducted in large tupperwares, garbage bins, coolers, etc.
Note that the tub-rub is commonly used as a form of payment.
Troy: "I deleted pretty much all your taboo porn."
Will: "What the hell!? That's mad retarded of you to do."
Troy: "I'm sorry man. Listen... I'll give you three tub-rubs to make it up to you."