HTTP status codes are grouped into five classes by their first digit: 1xx (informational), 2xx (success), 3xx (redirection), 4xx (client errors), and 5xx (server errors). The most important for SEO and performance are:

200 OK — request succeeded; page is crawlable and indexable.
301 Moved Permanently — canonical redirect; passes link equity to the new URL.
302 Found — temporary redirect; does not pass link equity permanently.
404 Not Found — page missing; crawl budget is wasted and any inbound links lose value.
410 Gone — page permanently removed; tells crawlers to de-index faster than a 404.
500 Internal Server Error — server-side failure; prevents crawling entirely.

Why it matters: Broken links, redirect chains, and server errors all hurt crawl budget, indexation, and user experience. Monitoring status codes across your site surfaces these issues before they affect rankings.

Quick tips:

Prefer 301 over 302 for permanent moves so link equity transfers correctly.
Use 410 instead of 404 when you've intentionally removed a page.
Fix redirect chains (A→B→C) into single hops (A→C).

See also: Redirect, Crawl Budget, Robots.txt.