Why is LED lid when pin is set low? I made this simple piece of code, to make my LED flash.. Which it does when the pin is set low.. But how does that makes sense?

#include "pins_arduino.h"

void setup() {
  // put your setup code here, to run once:
  pinMode(BUILTIN_LED,OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  digitalWrite(BUILTIN_LED,LOW);
} 


I am using the Wemos d1 - http://www.wemos.cc/
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.