The 404 error page shows you the pages and files not found on your website. These can come from visitors on your website or search engine robots trying to locate a file.
If you are not certain what 404 errors are, and why they are bad for your website, read this article: What are 404 errors?
Where referrer information is available you can see in the column to the right where the link was from.
Once you have taken action on the not found content you can either choose to mark individual links and remove them from the list or use the “Reset 404 Errors” button on top of the page to delete all.
Note that some files are supposed to return 404 – not found. This can be from robots crawling for WordPress plugins that are known to have vulnerabilities and then try to break in.
When you have manually deleted a page on your website then consider setting up a redirect to other another URL related to the content, such as a product category for e-commerce products or the homepage itself.
You can either do this manually by editing your .htaccess file or use a plugin such as the great WordPress SEO plugin by Yoast or the very effective and easy Redirection plugin by John Godley.