404 Error is an indicator for Hypertext Transfer Protocol (HTTP) status code for the server could not find the requested website.
In easy language,
Your browser can connect to the website server but the specific page address user is looking for is not available.
When a user clicks on a deleted link or a link that is not in existence inside the server, the Website will show an Error Page And that error page is known as 404 Page.
It can occur because of Website migration, Link structure changes into Permalinks, mistyped spellings into page slugs, and Deleted or moved posts.
This error does not affect the SEO of the site. But when google crawls a page that is not in existence, it does not get indexed. This means having broken links may not affect the whole site SEO.