There are 2 error pages on every site.
The first one is a part of the normal content and is called 404 Error. This page is used when someone attempts to link to a page that doesn't exist on your site. For instance, if there is an old link or marketing url that used to exist on your site but no longer is active. By default, this page will use the 'Main' layout - it is generally a good idea to put a site map on that page so visitors can re-orientate themselves.
The second error page is in the root of your project files and is called error.htm.
Occasionally, your visitors might get a hard system error in which case you will want to upload an error.htm file to the root of your site. There is a sample one in your site already. This file does not process any code; it is a basic html file where everything is hardcoded in order to prevent an error loop, which would crash the server. Server errors can happen if a layout calls a pod location or content block that doesn't exist or if there are error in the site setup like shipping rates not existing.