What does this LED light mean?

I am trying to troubleshoot 5 desktop PCs of same model.
I realize that those working PC having the LED light "On" under wire beside the CPU fan.
Those unbootable PCs do not have this LED light "ON".
What does the "On" and "Off" status of this LED light on the motherboard?
Please see attach picture.

Thank you.

Attached Thumbnails