Your hairline is like a lollipop because every time someone licks it, it gets shorter.
Lollipop Hairline
Explanation
Explain Bear
Okay, so basically, the joke is saying that the person's hairline is receding, like how a lollipop gets smaller when you lick it. It's kinda like saying you're balding, but in a silly way. And you, the listener, probably have the same problem, ya baldy.