What is red light below spindle speed bar?

Hi,
I setup LinuxCNC and basic configurations are done.
I can home XYZ, Spindle can be turned on by CW button and stop. With M3 Sxxxx MDI code, I can change speed.
But the bar below doesn't do anything and the light on the right side of the bar is RED. I saw others from YouTube has it GREEN.
What does this button say and what is missing from my configuration?

Thanks for your help!
 

See the documentation: linuxcnc.org/docs/2.9/html/gui/gmoccapy....pindle_feedback_pins

Thanks for the good reference point!
I can connect it in HAL ini.