SSO reduces password fatigue for users having to remember a password for each application. With SSO, a user logs into one application and then is able to sign into other applications automatically, regardless of the domain the user is in in or the technology in use. SSO makes use of a federation services or login page that orchestrates the user credentials between multiple applications.
If you have signed into Gmail and noticed that you were also able to access Google portfolio apps such as Google Maps, YouTube, Google Play, Google Photos and other Google applications, you are already using SSO! The user logs in once to a Google account, and has access to other Google applications.