This is a typical RPM (rotations per minute) indicator found in a helicopter:
(Other types also exist.)
The needle on the left shows the engine RPM, that is how fast the engine is turning. The one on the right shows how fast the rotor is turning.
They are both marked as percentages of something defined by the manufacturer.
You’ll notice green (and red) areas on the two scales. “Rotor in the green” would mean, for this particular helicopter, that the rotor is spinning at between 90% and 110%.
You can fly if the engine RPM is not in the green, under specific circumstances, but it’s tricky to fly if the rotor spins too slowly, and definitely very dangerous if it starts to overspeed.
source: Alexa Deva