An informal description of a Security Token Service is that it is a web service that offers some or all of the following services (among others):
It can issue a Security Token of some sort based on presented or configured credentials.
It can say whether a given Security Token is valid or not.
It can renew (extend the validity of) a given Security Token.
It can cancel (remove the validity of) a given Security Token.
It can transform a given Security Token into a Security Token of a different sort.
Offloading this functionality to another service greatly simplifies client and service provider functionality, as they can simply call the STS appropriately rather than have to handle the security processing logic themselves. For example, the WSDL of a service provider might state that a particular type of security token is required to access the service. Then:
A client of the service can ask an STS for a Security Token of that particular type, which is then sent to the service provider.
The service provider could choose to validate the received token locally, or dispatch the token to an STS for validation.
These are the two most common use cases of an STS.
For more information about the usage of Security Token Service, please read the STS chapter in the Talend ESB Infrastructure Services Configuration Guide.