An Identity Broker is an intermediary service connecting service providers with identity providers. The identity broker creates a relationship with an external identity provider to use the provider's identities to access the internal services the service provider exposes.
From a user perspective, identity brokers provide a user-centric, centralized way to manage identities for security domains and realms. You can link an account with one or more identities from identity providers or create an account based on the identity information from them.