Wind tunnels are a perfect tool to help race teams as well as manufacturers to develop cars that are aerodynamic and can cut through the air. But, testing a stationary vehicle does not make much sense and this is where the rolling roads or car treadmills come in.
The name rolling roads or car treadmills as suggested by their name are treadmills for cars. There is a rolling strip of metal that sits flush with the floor. This allows the car to rotate its tires and drive in place while being tested in the wind tunnel. The engineers can incorporate an entire extra set of aerodynamic forces affecting downforce and deflection. This gives a much better on how the car will behave in an actual racing environment.
The video below put together by Ford Performance gives excellent detail on how the technology works.