Single Sign-On (SSO) is a system that lets people use the same login details for multiple platforms. An example you are probably familiar with is Google, which enables you to use the same details for all its services, including YouTube, Gmail, and Chrome.
It’s useful for community builders because you can let anyone with an account for your business register without creating new log-in details. For example, an ecommerce store can let people sign up for their community using the details they enter when shopping.
The main benefit is that not having to register for a new account removes one of the barriers to joining your community. This may improve acquisition.