Single sign-on (SSO) is an authentication process based on a user log into multiple systems with single username and password. It is used for systems that require authentication for each application while using the same credentials. SSO works on central service from where the user gains access to different applications without logging in again.
Example
Tom has a Google account into which he logged in and receives a Google Accounts-generated cookie. He wishes to enter Google Drive. After that, he is redirected to his Google account which sees that Tom already has an authentication-related cookie, so he is redirected to Google Drive.