A 404 error is the answer a website gives when someone asks for a page that does not exist. The number is the code the server sends back and the page you see is the message your site shows with it. Every broken link, deleted product and mistyped address ends in one, and each is a visit that went nowhere.
A few 404s are normal and harmless. The problem is a page that used to matter, has links or rankings and now returns 404 instead of forwarding to its replacement.
HTTP status 404 Not Found. Google drops a URL from the index after it returns 404 persistently, and it is the correct response for content that is gone with no replacement (410 Gone is a slightly faster signal). Content that has moved should return a 301 to its successor, never a 404.
A soft 404 is the opposite mistake: a page that says not found, is empty or redirects everything to the homepage, but returns 200 OK. Google flags these in Search Console and treats them as errors anyway. Make sure the not-found template actually sends a 404 header and is not a themed page served with 200.
HTTP/1.1 404 Not Found