Basically, it's having sexual intercourse inside the hide of a bear. It doesn't necessary need to be bear hide, but it's best if it is because as the male is ready to ejaculate, he can get off the woman, point his penis at the bear's head and yell out "TAKE THAT YOU FUCKIN' BEAR!!"
Friend #1: "Have you tried the thermal bear?"
Friend #2: "...no"
Friend #1: "You're missing out on some good shit then"