void setup() {
  Serial.begin(9600);
  pinMode(LED_BUILTIN, OUTPUT); // set the digital pin as output:
}

void loop() 
{
  if (Serial.available()) { // if there is data comming
    String command = Serial.readStringUntil('\n'); // read string until newline character

    if (command == "ON") 
    {
      digitalWrite(LED_BUILTIN, HIGH); // turn on LED
      Serial.println("Turn LED ON");
    } else if (command == "OFF") 
    {
      digitalWrite(LED_BUILTIN, LOW);  // turn off LED
      Serial.println("Turn LED OFF");
    }
  }
}