Accessed in Appearance → Customize → Page Settings → Maintenance mode.
When this feature is active your visitors will be redirected to page that you have selected in Maintenance Mode settings. However, this page will be displayed to not logged in visitors without theme header and footer.
This means that when you are logged as administrator, then you will see the whole page normally, while not logged in users will see only content.
A good example of such a situation can be seen here.
More about the subject you can read in the article on our blog.
404 errors or endless redirects for not logged in visitor
If you face 404 errors or redirects that crash your website, be sure to check if you don’t have active Maintenance mode.
Often after finishing development process, you may delete maintenance page without deactivating maintenance mode, and it will try to redirect the visitor to strange places 🙂
If you are facing 404 errors only on Albums or Works please check refreshing permalinks.