"What exactly are the updates it is referring to that would cause visitors not to view your website? When you are editing your website or posting new content, can people visit your website normally?"

People can still visit your website as you edit/post new content.  The Site Maintenance tool is a built-in feature you can use to redirect website visitors for when you want to restrict access, for example when you are upgrading or creating your website.  Some people like to do this, for example to redirect users to a "Coming Soon" page while you create your website, where you could also have a mailing list to then email them to announce your website launch.  You can choose to use this feature or not.