Knock Knock, The guy who finished second.
The guy who finished second.
The guy who finished second who?
This joke references the idea that nobody remembers the name of the person that finished second in a race. The idea was attributed to various people at different times, but this joke takes that adage, and turns it into a punchline - because the listener of the joke isn't expected to realize that it is referencing that particular saying until the punchline is delivered.
As far as knock knock jokes go, this one is pretty good. I am not saying that I would laugh at it, but it is cleverer than most.