What a soft 404 is
A soft 404 happens when the server says a page loaded fine (HTTP 200) but the content tells another story: "Page not found", "No results", an empty category or an almost blank template. Google detects the mismatch, labels the URL "Soft 404" in Search Console and keeps it out of the index.
A real 404 or 410 status is the honest answer for a missing page. A soft 404 wastes crawling because Google has to fetch and analyse the page to find out it is empty.
Common causes
Custom error pages that return 200 instead of 404; out-of-stock product pages with no content; empty search, tag or category pages; pages that render content only with JavaScript that fails for Googlebot; and deleted pages that 301 redirect to the homepage or an unrelated page.
How to fix a soft 404
If the page is gone, return a proper 404 or 410. If it moved, redirect it to the most relevant replacement, not the homepage. If it should exist, give it real, useful content and make sure it renders for crawlers. For empty filter or search pages, return 404 or add noindex.
After fixing, use Validate Fix in the Search Console page indexing report.