When you suck really hard on someone’s skin, it can draw a lot of blood near to the surface, causing a bruise-like mark, usually around the neck. Sometimes they’re even mouth-shaped. ;) It means they were really heavy into the making out. It’s physical evidence of something that’s supposed to be a secret, I guess, but when you get carried away, you might not care during the making. And if your friend is young enough, and his parents not very open-minded, he might get in trouble like I did.