So...yeah. It's basically a joke about going commando. Honestly, though, that position she's in disturbs me more than the naughty underwear-related one-liner. That looks really, really uncomfortable. It looks even more uncomfortable when you realize that she's just sort of...sitting there and holding that pose. Like a creepy doll. I look at it and can't help but think of that artist who got random women to imitate weird, unnatural fashion model poses in real life and secretly filmed the reactions of people passing by.
I like this other 1980s Calvin Klein ad featuring Brooke Shields better. Some of those weird poses are still there, but at least she gets to move around and give a smart-person speech. Also, I'm a sucker for a good pun. (Kinda wish she'd kept the glasses on, though.)