The server refuses to brew coffee because it is, permanently, a teapot.
HTTP 418 I'm a Teapot was defined in 1998 as an April Fools' joke in RFC 2324 (Hyper Text Coffee Pot Control Protocol). While not a real HTTP status, it has been implemented by many servers as an Easter egg and is referenced in various HTTP libraries.
No SEO impact. If your site returns 418, search engines will treat it as an error.
If LemWatch detects a 418 from your production site, something has gone hilariously wrong (or delightfully right).