FOUR OH FOUR!
Everybody on the internet knows what a 404 is, but where does that number come from? It is an HTTP status code, part of the protocol that allows web servers to communicate with your browser. Every piece of information passed between server and client has a code. Usually, it is a ‘200 OK’, which just means that the request was successful.
Status codes in the 400 range correspond to client errors. Thats where the server is operating correctly, but you did something wrong. In the case of ‘404 Not Found’, you asked the server for a thing that it doesn't have. (Odds are I gave you a broken link! Let me know, please.)
Most of the codes are standardized, but some developers will use unconventional numbers for fun. If you make too many requests on the Twitter API, they'll send you a '420 Enhance Your Calm' to tell you you're being rate limited.