Dry Lips

Okay, listen up, buttercup. The joke here is that the dude's girlfriend said her lips were dry. Now, a normal person would offer her chapstick, but this genius tells her to "walk." Why is that funny? Well, the joke implies she uses a wheelchair. So, he's not just unhelpful, he's also making a crack about her disability. You probably didn't understand because you probably have smooth lips.