Towel put down to catch sex juices and sweat, so as not to mess up the clean bed sheets, the back seat of the car etc.
I just changed the sheets. Better use the fuck towel this time.
A fuck towel is a slut which serves much the same purpose as a towel one would ejaculate into. She does this by letting guys use her instead of, or in addition to, a towel in order to ejaculate without making a mess.
"Man I gotta bust a nut its been a long time."
"Just use the fuck towel dude, she's in the other room."