Single Sign-on is a method that allows you to sign on with the same login credentials across multiple systems. You have probably already seen it a lot of places on the internet, where you can sign on to almost any service using for example your Google or Microsoft accounts.
Once set up, only users with an email on your SSO domain will be able to log in. For example: if your SSO is setup to @naboo.com then only users with an @naboo.com email can log in to your Openli account.
When enabled, users will be forced to sign in using their SSO account.