Why is LED lid when pin is set low?
If the LED is ON when LOW and OFF when HIGH then chances are the LED is connected between +3.3V and the IO pin with a suitable resistor.

When HIGH the IO pin is at +3.3V, so the voltage across the LED is 0V (3.3 - 3.3 = 0) and it is turned off.  When LOW the IO pin is at (nearly) 0V and the voltage across the LED (and its resistor) is 3.3V (3.3 - 0 = 3.3), so it lights up.