What a broken link is
A broken link, or dead link, sends visitors to a URL that fails: a 404 "Not Found" or 410 "Gone" response, a 5xx server error, a timeout or a domain that no longer resolves. Internal broken links point to your own pages; external ones point to other sites.
They appear when pages are deleted or renamed without a redirect, when URLs are mistyped, when another site restructures, or when a domain expires.
Why broken links matter for SEO
For visitors, a broken link is a dead end that erodes trust. For search engines, internal broken links waste crawl activity and break the paths that spread link value through your site, and backlinks pointing to dead pages pass nothing to the rest of the site.
A few broken external links will not tank rankings, but a site with many looks neglected and loses the value of links it already earned.
How to fix broken links
Update internal links to point at the correct live URL. If a page moved, add a 301 redirect from the old URL to the closest replacement, so backlinks and bookmarks keep working. For dead external links, link to an updated source or remove the link.
Make sure your 404 page returns a real 404 status; an error page that returns 200 is a soft 404. Recheck after migrations and content clean-ups, when most broken links are created.