so, in the joke, the guy was working at the keyboard factory, right? and he got fired because he made a big mistake - he left out one key from the keyboard, the letter "f". so the f key wasn't there, so what did he make? a "rookie" mistake, but i fixed it by adding the missing "f", which made it "funny" instead of "rookie" - a pun! the joke is that it's a play on words, because he made a "rookie" mistake.