The login page setting is used to create a customized login experience by replacing the regular wp-login.php page with a branded page that you have control over.
This page set will replace the default wp-login.php page (this is the default WordPress login page). This includes the admin user when logging in to /wp-admin.
Before you configure this setting to point to a page, make sure that the page has our [memb_loginform] shortcode on it. Otherwise, you will be unable to login to your site.
If you accidentally set this without a login form, you can still access your regular login form by going directly to your login page with the ?action=override parameter.