Every Mendix object has a unique, immutable id attribute. This attribute is hidden in the domain model to make the model more legible and less cluttered. Referring to the id is a simple way of saying you want that specific object and no others.
The [%UserRole_<user role="" name="">%] token is automatically generated by the modeler for every user role in your model. Every user role in the model, both default and those created by a developer, will generate such a token. These tokens allow developers to refer to user roles in UI constraints without having to resort to needlessly convoluted constructions.
Every Mendix object has it's own id. For user roles you have extra tokens to retrieve a specific user role. The XPath above retrieves the user role Employee so you can add for instance this user role in a microflow to an account.