Every HTTP response starts with a status code grouped by the first digit: 1xx informational, 2xx success, 3xx redirect, 4xx client error, 5xx server error.
The well-known ones, 200, 301, 404, 500, cover 80% of what you'll see. The rest add nuance: 401 vs 403 (auth vs permission), 422 (validation failed), 429 (rate limited), 503 (overloaded).
Returning the right status code matters for SEO (Google trusts 200s), caching (CDNs cache 200s and 301s differently), and API consumers (clients branch on 4xx vs 5xx).