A 404 error is the HTTP status code a server returns when a requested page doesn't exist — either because it was never there, was deleted, or its URL changed without a redirect being set up.
The "404" is one of many standard HTTP status codes (200 means success, 301 means a permanent redirect, and so on), specifically meaning "not found". When a browser or crawler requests a URL and the server has nothing to return for it, a 404 response is the correct, expected behaviour — it's not inherently an error in the sense of something being broken, just an honest "this doesn't exist" answer. 404s become a genuine problem when they happen unintentionally — a page that used to exist and had traffic or backlinks pointing to it, but was removed or moved without a redirect left in place. Visitors following an old link or bookmark hit a dead end, and any ranking value or backlink equity the old page had is generally lost rather than passed on. A well-designed custom 404 page (as opposed to a generic server default) can help recover some of that lost visitor by offering navigation back into the site, but it doesn't restore the specific page's own ranking history.
Unintentional 404s on pages that used to have real traffic or backlinks represent lost value that a simple 301 redirect to a relevant replacement page could largely have preserved — checking for these after any site restructure or URL change is one of the highest-value, lowest-effort technical SEO habits available.
site audit flags broken links across your site as part of every crawl, including internal links pointing at pages that now return a 404 — giving you a prioritised list to fix rather than discovering them one at a time from frustrated visitors.
Not inherently — some 404s are expected and harmless, like a mistyped URL or a link to genuinely deleted content with no replacement. The problem is specifically 404s on pages that used to have real traffic, rankings, or backlinks and were removed without a redirect.
Set up a 301 (permanent) redirect from the old URL to the most relevant current page, if one exists. This preserves most of the ranking value the old page had built up and sends visitors somewhere useful instead of a dead end.
Illumae's site audit crawls your site and flags broken links — including internal links pointing to pages that return a 404 — as part of its health check, giving you a prioritised, page-by-page list to fix.
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