The first digit gives a broad category: 2xx success, 3xx redirection, 4xx a request the server cannot fulfil as sent, and 5xx a server-side failure. That does not mean every 4xx is the user's fault or that every 5xx is safe to retry blindly.
Read the method, URL, response body, relevant headers, and redirect chain. 401 commonly means authentication is needed; 403 means access is refused; 409 signals a conflict; 429 calls for rate-limit handling. A 5xx after a non-idempotent request may occur after the action succeeded, so check state or use an idempotency key before retrying. The HTTP status reference gives the individual meanings.
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