A 404 means the server looked for the page at the requested URL and couldn't find anything there, the most common way a website tells a visitor a link led nowhere. It happens when a page is deleted without a redirect, a URL changes during a site restructure, or someone simply mistypes an address.
A handful of 404s is normal on any active site; content gets removed, structures change. The problem is scale and visibility: too many 404s waste crawl budget on dead ends, and a 404 on a page that used to rank and get real traffic represents lost value that a redirect could have preserved. A well-designed 404 page helps too, offering a search bar or links back to popular sections rather than leaving a visitor stranded, but the better fix wherever possible is heading it off with a 301 to the closest live equivalent before the old URL disappears.
