What are roundabout metering traffic light signals?

What do they look like? What is their function? What colours do they have? What was the original changeable message sign which operates along with them which rotated before and after operation? What did the sign say when the lights were off and what did it say when they were on? I also want to know what the advanced warning sign with the flashing yellow lights said when the lights were in operation and when they were not in operation.