The server can not find the page you are looking for. It might have been deleted, the URL was typed wrong, or the page was moved without a redirect.
A visitor clicks an old link from another website that points to a page you have since removed.
A 404 wastes Google's crawl budget. If an important page returns 404, Google will eventually deindex it and you lose all its rankings. Broken internal links also dilute PageRank. Always redirect deleted pages to their closest equivalent.
Set up 301 redirects from deleted URLs to relevant pages. Check Google Search Console Coverage report for 404 errors. Use the broken link checker to find internal 404s before Google does.
Check how any URL responds in real time