Single sign-on (SSO) is an identification system that allows websites to use other, trusted sites to verify users. This frees businesses from the need to hold passwords in their databases, cuts down on login troubleshooting, and decreases the damage a hack can cause.
SSO systems work as an identity provider, similar to an ID card. For example, if you are asked to provide ID checking into a hotel, the receptionist doesn’t have to know you personally, they can just look at your license or passport and see that your state vouches for your identity.
Likewise, with SSO, your website doesn’t make you prove your identity by checking within itself. Instead, it checks with an SSO provider (such as LinkedIn, Microsoft, or Google) to see if it can verify your identity. If it can, the site takes their word for it.