Either (1) a designation on which security information can be applied, or (2) an organizational term meaning a collection of responsibilities.
In the former sense, security roles interact with user groups to allow the assignment of permissions.
In the latter sense, the role of administrator might be assigned to Bob, who also has the roles of power editor and approver. The usage of the term “role” abstracts a collection of responsibilities away from a specific person. Responsibilities belong to the role not the person, though a specific person might hold that role at any given time.