429 Too Many Requests
Meaning
The user has sent too many requests in a given amount of time (rate limiting). The server is throttling the client, typically with a Retry-After header to indicate when to try again.
When to Use
When a client has exceeded the rate limits defined by the server or API. It indicates the client should slow down or stop sending requests for a while.
Example
An API allows 100 requests per minute; if a client sends the 101st request within that minute, the server may respond with 429 Too Many Requests, possibly including a Retry-After header telling the client when it can resume.
When Not to Use
Do not use for general throttling without communicating limits. Ensure that this is used in the context of rate limiting. If the error is due to something else (like a single request being too large or malformed), use a different code.
Source
RFC 6585