Sexual activity, specifically oral sex, between two people.
Swapping cunts, like most sexual slang terms, can mean any number of other acts. They include, but are not limited to: Making out, Vaginal intercourse, Anal intercourse, manual stimulation, fisting, 69, or any variety of shit play.
It is commonly said in a guido accent, as "Swappin' Cunts."
"You swappin'/swapping cunts in there?"
"He's with Kate... I think they're swappin' cunts."
(To the tune of "The Love Boat")