You need to have Assign Agent Rules add-on installed in order to assign agents automatically depending on conditions set. Automatic assignment of ticket is not possible in free version.
Agent Role is set of permissions an agent can have of this role. Administrator role in Agent Role does not mean it is WordPress website role, it is role having all permissions enabled for an agent.
You can add an agent in Support Agents submenu.
Agent is user who reply tickets on behalf of the website owner.