404 means that a web sever doesn't have something at the URL you requested.

Search engines build an index periodically by looking at what's on a website. That's what provides your search results. They aren't informed if a web server removes or moves content, though eventually, they'll have a bot that checks, notices the change, and updates the index.

They

well, Google, and I assume most

will update major sites more frequently, so this is probably more common if you're pulling something off a small, backwater website.