What did Dumbledore say his scar on his left knee was?

In Philosopher's Stone, we hear Dumbledore replying to McGonagall's query as to whether he could do something about the scar baby Harry has on his forehead:

“Even if I could, I wouldn’t. Scars can come in handy. I have one myself above my left knee that is a perfect map of the London Underground." - Chapter 1, The Boy Who Lived.

Now, I have no idea what that last sentence means.

It could be because English is not my native tongue. Is it some kind of a joke, or does it really mean something?