Is this on the card or the motherboard ?
Either way, you could consult the card/mobo manual, or visit the manufacturers website(e.g. asus, xfx, gigabyte, etc).
This is from a Gigabyte RX 580 GPU card. The LED on the card indicates the status of the GPU card's Fan.
3D ACTIVE FAN WITH LED INDICATORS
The semi-passive fans will remain off when the GPU is under a set loading or temperature for low power gaming. The LED fan indicator on the top of the graphics card provides an instant display of the fan status.
This is from just one Manufacturer. Need to go to your Manufacturer's GPU card and it will tell what the LED is for. Most likely is similar to Gigabyte's LED indication.