The 404 error means that the url a user entered cannot be found on the server. Usually these pages are white ones with black letters saying that page not found.
jCore comes with a customized 404 error page where it's content can be 100% customized from the Global Settings and the error will appear in the content of your website this way giving the users a possibility to click on the menus/links they may have wanted to see but misstyped the url instead of showing a white page and giving the impresion that you page doesn't exists at all.
The default custom error text looks like this but you may customize to your needs from the admin.
Address Not Found /features/custom-404-error-page/custom-404-error-page?
Unfortunately the page you are looking for can't be found on the server jcore.net.
Check the spelling of the address you typed.
Are you certain this page exists? The link may have expired.
If you are still having problems, please visit the Main Page.
With the custom 404 error page you attract more visitors to your site and give them more opportunity to find the right content they are looking for not to mention that expired / removed links from your site may attrack further visitors too as they won't be shown a white page and search engines will also recognize them as real pages.
NOTE: Custom 404 Error page works only with SEO Friendly Links activated.