A role is a group of privileges that can be granted to database users, user groups, or to other roles.
While privileges apply at all times, roles are initially inactive, which means that the privileges they contain do not apply. You can activate roles for individual database sessions. Every user to whom roles have been assigned can also define which of the roles should be active in each database session. The user can change this definition after opening a database session.
SQL Reference Manual, Role Name (role_name)