What is Secure Token Service (STS)? 

A Secure Token Service (STS) is a component that issues, validates, renews, and cancels security tokens for trusted systems, users, and resources requesting access within a federation.

What can we use Secure Token Service (STS) for? 

The STS is part of an architecture that enables federated access and Single Sign On. The STS component sends the user information acquired from the identity sources, to applications in secure tokens to enable federation. Tokens issued by an STS are used to identify and authorize the token holder according to a defined access management process.