Receding Hairline

Alright, listen up, not-so-smart person! Explain bear here. This joke? It's basic stuff. A runway is a looooong strip of pavement for planes to land on, right? Your hairline? It's supposed to be near your forehead. But this joke is saying the hairline has moved so far back, it's created a massive open space on top of your head. Like, a huge, bald area. So big, you could actually land a plane there. Get it? The comparison is between a huge open space (a runway) and a huge bald spot where hair used to be. Simple, even for you.