Arduino LED actuator
Perhaps "actuator" is a mis-translation?  I would call the LED an indicator.

When debugging a program on a microcontroller, it is often handy to have a LED that know will work to use as an indicator that the program has reached a certain place, or attempted to do some action. Eg: set the LED on when a motor should start, and off when it stops - if the LED works, but the motor doesn't, you might start looking at the wiring for a problem...