…even after all these years.
If you're too lazy to watch it, please enjoy this intriguing description of the scene, as provided by wikipedia:
The premise of the routine is that Abbott is identifying the players on a baseball team to Costello, but their names and nicknames can be interpreted as non-responsive answers to Costello's questions. For example, the first baseman is named "Who", so that the utterance "Who's on first" is ambiguous between the question "Which person is on first?" and the answer "Mr. Who is the name of the first baseman."
Seriously though, the wiki is actually pretty interesting.