A 404 page is the page you see if you ask for a page that no longer exists (or never did) on a website. This happens when a page that you had bookmarked is deleted or moved, or if you type in a page URL wrong.
The advantage of a customized page is that you can be more polite than the standard 404 page and also offer the reader some options for finding the right page.
Here’s the Fast Smart Web Design 404 page, for example:
In the last workshop, someone asked if you could create a custom 404 page on a Weebly site.
It doesn’t look like it. However, readers do get a Weebly 404 page that has a link taking them back to the home page: