Sometimes when you open a webpage, instead of normal content you get a string of numbers and a few words thrown at you. These are 'status codes' — the server's way of telling your browser 'here's what just happened with that request.'
The most common one. It means the server can't find any content matching the URL you requested — usually because the address was typed wrong, or the page has been deleted or moved somewhere else.
This one has nothing to do with you — something broke inside the website's own code or server process, and it couldn't handle the request. Usually refreshing after a bit, or waiting for the site to get fixed, resolves it.
502 usually shows up when something goes wrong as one server passes a request along to another. 503 generally means the server is under heavy load or undergoing maintenance and can't handle your request right now — it's usually fine after a short wait.
If you hit a status code you don't recognize, an HTTP status code lookup tool gives you its name and a one-line explanation instantly — no need to go searching the web every time.
I counted the data behind the HTTP status code reference tool. It contains 19 common status codes in five categories: 2 informational (100, 101), 3 success (200, 201, 204), 3 redirection (301, 302, 304), 7 client error (400, 401, 403, 404, 405, 408, 429) and 4 server error (500, 502, 503, 504). It is not a complete list of every standard code, so 418, 307 or 410 will not be found, and the tool then shows a message saying no matching status codes were found. Here is how to read it with a code that does exist. Type 404 in the search box and you get one card: the number 404 in the client-error color, the name Not Found, the Chinese name for it in the Chinese interface, and a one-line description saying the server could not find the requested resource and that it is one of the most common error codes. The search matches the code number, the English name or the Chinese name, so typing gateway lists both 502 and 504, and typing 40 lists six codes from 400 to 408.
Next time one of these numbers pops up, don't panic — most of the time it's just a temporary hiccup on the site's end. Give it a refresh in a bit and it'll usually sort itself out.