SSO stands for Single Sign-On and allows user to login with tools you use everyday.
For example, you can login or register on Signitic with your G Suite account or your Office 365 account.
This process uses the security system of the third party application and so can use more advanced security mesures like Double Factor Authentification.
This option, when activated, will disable basic connection with email/password. So it will ensure that your Signitic administrators use SSO connection when login.
This will increase then the global security of your account and informations.