// Warning : This is a bad example ! I don't like it.

String myInputString;

void setup() 
{
  pinMode( 13, OUTPUT);
  Serial.begin( 115200);
  Serial.setTimeout( 100);        // not failsafe, because input stream is unknown
  Serial.println( "Type ON or OFF for the led");
}

void loop()
{
  if( Serial.available() > 0)
  {
    myInputString = Serial.readString(); // waits for more input and stops with the timeout

    myInputString.trim();                // get rid of Carriage Return and LineFeed
    myInputString.toLowerCase();         // everything to lowercase, to allow upper and lower case

    if( myInputString == "on")
    {
      digitalWrite( 13, HIGH);
    }
    else if( myInputString == "off")
    {
      digitalWrite( 13, LOW);
    }
  }
}