2xx range indicate Success; the request is successfully received, understood, and accepted. Whereas, codes in the 4xx range indicate a client error — the request contains bad syntax or cannot be fulfilled. Errors in the 5xx indicate a server error — the server failed to fulfil an apparently valid request. For generic knowledge on the status code, see this topic.
Here is a generic list of error codes and what they imply in the context of Pleo: