"The 404 or Not Found error message is a HTTP standard response code indicating that the client was able to communicate with the server, but the server could not find what was requested." In simple words, 404 error means a page that does not exist on the website. This page normally appears when someone mistypes the page URL or the webmaster himself deletes a page and someone tries to access that removed page.
Earlier, Blogger use to display a blogger-branded page on 404 error pages. But now, blogger has allowed to display your own text on 404 pages, you can even design the complete page for 404 error pages. By Default, Error page will display this error message.
You can change the text of this error page to something else by navigating your browser to;
Settings > Search preferences > Custom Page not Found
You can also use HTML codes to make your error template more attractive and interactive.