A role is a group of privileges that can be assigned to database users, user groups, or other roles.
While privileges apply at all times, roles are inactive, which means that the privileges they contain do not apply. Roles can be activated for individual database sessions. Users to whom roles have been granted can also determine which of the roles they want to be active in each of their database sessions. This decision can be changed after a database session has been opened.
See also:
Reference Manual, Role Name (role_name)