https://www.cisco.com/c/en/us/support/docs/security/email-security-appliance/118796-technote-esa-00.html

Explanation
Two-factor authentication adds a second layer of security to your online accounts. Verifying your identity using a second factor (like your phone or other mobile device) prevents anyone but you from logging in, even if they know your password.
Note: Single sign-on (SSO) is a property of identity and access management that enables users to securely authenticate with multiple applications and websites by logging in only once with just one set of credentials (username and password). With SSO, the application or website that the user is trying to access relies on a trusted third party to verify that users are who they say they are.