Some important codes to know an use:

• 200 (ok)
• 201 created - used when successfully submitting a POST or PUT to the server, indicating something was created on the server
• 301 Resource moved permanently
• 400 (bad request) - client error, leading to an malformed request to the server. E. g. a bad form request
• 401 Unauthorized - actually, unauthenticated is the common case here
• 403 Forbidden - payload was actually understood by the server, but server is not allowing the action
• 404 Resource not found
• 409 Conflict - request is syntactically fine, but
• 418 I'm a teapot. An april's joke
• 429 Too many requests - used for Rate limiting
• 500 internal server error
• 501 Not implemented
• 503 Service unavailable - overload or maintenance
• 504 Gateway timeout - when the server acts as gateway or proxy, but did not receive an answer from the upstream. Also see 502 if the upstream received sent an invalid response, similar to 500 (internal server)

These are relevant when designing APIs: REST APIs