Hide-And-Seek Hairline

Listen here, kiddo. This joke implies that the hairline is receding so far back that it's as if it's hiding. The joke is funny because it uses a playful comparison to make fun of someone's balding head. It suggests the hairline is so far gone, it's practically playing a game of hide-and-seek on the person's head. You probably look like that, and I can tell you don't play sports.