Hi,
what is custom 404 error page?

Hello,
In tech-speak, 404 is the error number returned when a visitor attempts to access a page that doesn't exist on your site. The two most common reasons a visitor gets a 404 error are:

They mistyped the Web address (URL) of the page they were trying to reach; or
They followed a link to a page that no longer exists on your site.

The Error 404 "Page not found" is the error page displayed whenever someone asks for a page that's simply not available on your site. The reason for this is that there may be a link on your site that was wrong or the page might have been recently removed from the site. As there is no web page to display, the web server sends a page that simply says "404 Page not found".

Custom 404 error is occured when a visitor attempts to access a page which is not available on your website. Visitor gets 404 error due to two most important reasons:
1. When the (URL) is mistyped they were trying to view
2. They entered to a link of a webpage that no longer exists on your site.

 A 404 page is what a user sees when they try to reach a non-existent page on your site because they've clicked on a broken link, the page has been deleted, or they've mistyped a URL.