Single sign-on means being able to access all of the applications and resources that you need to do business, by signing in only once using a single user account. Once signed in, you can access all of the applications you need without being required to authenticate (e.g. type a password) a second time.
Single sign-on (SSO) makes it easy for users to get started with an application. For enterprise applications, support for SSO is critical: many corporate security policies require that all applications must use approved SSO methods.