Basically, Soft 404 is a page that looks like a 404 but returns a HTTP status code 200. I mean, the website might show a Page Not Foundâ€-style page, but search engines such as Google will see the page and think that it's an actual live page.
How to fix:
You can use Google Webmaster Tools which will tell you about the ones Google has detected. It is controlled through your website's .htaccess file.
In order to configure your 404 page, simply add this line to the site's .htaccess file:
ErrorDocument 404 /cool404.html