The server is a teapot and refuses to brew coffee.
418 I'm a Teapot is the most famous joke status code on the web. It comes from RFC 2324, the Hyper Text Coffee Pot Control Protocol, an April Fools' RFC from 1998. Some servers return it for fun; some bot-detection systems use it as a tarpit response; it has been removed from major browsers' implementations and added back multiple times due to internet outcry. It will probably outlive us all.
When servers should return it: Return 418 only as a joke or easter egg. Real APIs should not use it.
curl -i https://www.google.com/teapot
HTTP/2 418
content-type: text/html
<html>I'm a teapot.</html>
For a one-page reference of all HTTP status codes, see the HTTP cheat sheet. For testing API responses, try the API Tester tool. For inspecting responses on the command line, the curl cheat sheet covers the most common flags.