404 error
A 404 error is the reply a server gives when the page someone asked for doesn't exist at that address.
A 404 error is what your server replies when someone asks for a page that isn't there. The address might be mistyped, the page might have been deleted, or it might have moved without a redirect. The visitor, person or bot, gets a "not found" page instead of the content they expected.
Some 404s are normal. Every site has them. They become a problem when they hit pages people still link to, pages that used to bring in visitors, or pages AI platforms try to read.
What search engines do with a 404
Google doesn't index pages that return a 404. If a page it already had starts returning one, Google drops it from its index, as its guide to HTTP status codes explains. Any links pointing at that address stop doing anything for you.
404s and AI answers
AI bots fetch pages the same way. A bot that gets a 404 has nothing to read, so it can't use that page in an answer. And if an answer links to an old address of yours that now returns a 404, the person who clicks it lands on an error instead of your page.
For example, Harrow & Pine moves its oak table range from /oak to /tables/oak and forgets the redirects. A furniture blog that AI answers often cite still links to /oak. Bots that follow the link find nothing, and readers who click through from an answer find an error page.
What to do about them
If the page moved, add a 301 redirect from the old address to the new one. If it's gone for good and there's no close replacement, a 404 or a 410 is the honest answer. Either way, make your "not found" page helpful, with a search box and links to your main sections.
Don't send every missing page to your homepage. A visitor who wanted a product and lands on the homepage is no better off.
404s in AnswerOrbit
Once you connect your server logs, the Bot visits tab in Website shows the pages AI bots read and which requests failed. Recommendations includes fixing pages on your site that return an error to bots. See Website and AI traffic and Recommendations.