Used to describe the act of having sex. Most commonly used in reference to hitting it from behind. Literally, having sex with butt cheeks.
Dude #1 : Hey bro, did you end up hooking up last night ?
Dude #2 : Oh yeah man. I took home a dime and I rocked those cheeks all night.
Dude #1 : Are you hanging out tonight ?
Dude #2 : No man, your sister is coming over tonight and she wants to rock cheeks.
Dude #1 : oh cool, wait, what the fuck ?