int ledPin = 9;
int adcValue; // adcValue

void setup() {
  // put your setup code here, to run once:
  pinMode(ledPin, OUTPUT); // LED pin is output
}

void loop() {
  // put your main code here, to run repeatedly:
  adcValue = analogRead(A0); // turn analog of A0 to digital
  // Map analog to the 0-255 range, and works as PWM duty cycle of ledPin port
  analogWrite(ledPin, map(adcValue, 0, 1023, 0, 255)); 
}