Status codes come in families keyed by their first digit. The digit tells you who is happy and who has a problem. You can react to the family even before reading the exact code.
2xx means success: the request worked, like 200 OK or 201 Created.
3xx means redirect: look elsewhere, the resource moved or is unchanged.
4xx means client error: your request was bad, like 404 Not Found or 401 Unauthorized.
5xx means server error: the server broke while handling a valid request.
This split guides retries and blame. A 4xx usually will not improve if you retry the same request. A 5xx often will, since the fault sits on the server side.
Rewriting in plainer words…
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