404 Error
A 404 is the HTTP status code returned when a requested URL does not exist. Some are normal and healthy on any site. They become a problem when they hit URLs that have inbound links, internal links or historical search rankings behind them.
Why it matters for your rankings
Google's position is that 404s are a normal part of the web and do not harm a site by their existence. That is true, and it is also frequently misapplied as a reason to ignore them entirely.
The cost is specific rather than general. A 404 on a URL with external backlinks throws away that link equity completely. A 404 that visitors reach from search or internal navigation loses a person who was already interested. A large volume of 404s from a botched migration wastes crawl budget and signals neglect. The correct response is triage, not panic: fix the ones with links or traffic, redirect what has an obvious successor, and leave genuinely dead URLs to return 404 or 410 honestly.
How to check it on your site
List 404s with backlinks
In Ahrefs open Best by Links and filter to 404 status. These are the highest-value fixes on the site.
Check Search Console
Under Indexing then Pages, review 'Not found (404)' for URLs Google is still trying to crawl.
Find internal links to dead URLs
Crawl with Screaming Frog, filter Response Codes to 4xx, and use Inlinks to locate the source pages.
Make the 404 page useful
Include search, navigation and popular links so the visitor has somewhere to go rather than leaving.