It indicates the state of the +12V power supply. When the LED is on, the AutoTuner is receiving power from a +12V source. When flashing, it means that at least one of its +12V outputs is on. In that case be extremely careful because there is a risk of short circuit with a metal object if the power wires are not secured correctly.
It flashes to indicate that the interface is working properly. If it does not flash, it is recommended to unplug and restart the interface.
USB activity (orange)
It flashes when data is transferred via USB.