Ground - Cleared for Takeoff.
Air - Cleared to Land.
If the aircraft is in flight, a steady green light signal from the control tower means cleared to land. If the aircraft is on the ground, it means cleared for takeoff. If the light is aimed at ground vehicles or foot traffic, steady green means cleared to cross the runway, or proceed.
A steady (instead of flashing) yellow light means the traffic signal is changing from green to red. You must stop if it's safe to do so. If you're already stopped you must not proceed.
A red light - steady or flashing - means STOP. A steady amber light means Caution - signal about to change. A flashing amber light means SLOW DOWN.
If your aircraft is on the ground, a steady red light from the control tower light gun means STOP. If you are airborne, a steady red light means to give way to other aircraft, and continue circling.
A traffic signal displaying a steady circular yellow light means that you should use caution when going through the intersection. You have the right away, but watch for other cars.
A traffic signal displaying a steady circular green light means that, at least according to the light, you have the right to proceed. BUT, that also means "only if it is safe to do so". For example, a driver coming South is in the intersection, going straight, when the light changes. Traffic stops him from getting out of the intersection. You are traveling west and want to turn South. But his Range Rover is in the intersection! You have the green light! What you do is stay put, even if your light changes to red before you can move into your turn, even if everyone behind you gets mad at you. You must not clog up the intersection, because drivers coming East may want to turn North.
Exercise extreme caution
It means that oncoming traffic is stopped by a red light.
General Warning: Exercise Extreme Caution.
Exercise extreme caution.
It simply means that the light's been green for a time already, as opposed to a fresh green light, which just turned green.
Flashing RED: clear the active runway. Flashing RED AND GREEN: General Warning: Exercise extreme caution.