What is a Relying Party? 

A Relying Party (RP) refers to a partner in a federation that consumes security tokens for providing access to applications. The Relying Party will delegate access requests to a third-party system such called an security token service (STS) that is acting as an Identity Provider (IdP) to enable single sign-on (SSO) for end users.

What can we use a Relying Party (RP) for? 

An STS can behave as an Identity Provider or a Relying Party depending on whether it’s receiving or sending security tokens containing user information in that instance. The RP relies on the other service for authentication before allowing users, devices, or services access.