Identity brokering is a way to establish trust between parties that want to use online identities of one another. Over the years we have developed many standards for doing this like SAML, OpenID, OAUTH, OpenID Connect but the problem is that very few people understand how these protocols work and where are they supposed to be used. It gets complicated to implement such protocols and is also expensive and time-consuming.
Identity Broker Service
An Identity broker service hides all the complexity of these protocols and provides a simple HTTPS endpoint for parties to use. Without implementing SAML, OpenID, OAUTH or OpenID Connect, you can suddenly start speaking these languages and have access to identity and access tokens from hundreds of providers. The only thing you will need to know is how to call an HTTPS endpoint which is much simpler than understanding different standards.