Every HTTP response has a status code. browser, optimistically: GET /cat.png (request)
server, sadly: 404 not found (404 is the status code!)

There are 50ish status codes but these are the most common ones in real life:

2xxs mean ★Success★ - 200 OK

3xx s aren’t errors, just redirects to somewhere else - 301 Moved Permanently - 302 Found: temporary redirect - 304 Not Modified: the client already has the latest version, “redirect” to that

4xx errors are generally the client’s fault: it made some kind of invalid request - 400 Bad Request - 403 Forbidden: API key/OAuth/something needed - 404 Not Found: we all know this one :) - 429 Too Many Requests: you’re being rate limited

5xx errors generally mean something’s wrong with the server. - 500 Internal Server Error: the server code has an error - 503 Service Unavailable: could mean nginx (or whatever proxy) couldn’t connect to the server - 504 Gateway Timeout: the server was too slow to respond