From the Class UserAgentApplication documentation there is a reference to STS. What is STS?
acquireTokenSilent: Used to get the token from cache. MSAL will return the cached token if it is not expired. Or it will send a request to the STS to obtain an access_token using a hidden iframe. To renew idToken, clientId should be passed as the only scope in the scopes array.
STS stands for the Security Token Service. In the context of the standard OAuth protocol, it basically maps to the Authorization Server which issues tokens to the applications after authentication and authorization.
In Azure AD B2C, the STS can federate with other Identity Providers such as Facebook, Google, etc.
来源：https://stackoverflow.com/questions/45174728/what-does-sts-mean-in-the-context-of-azure-ad-b2c