int counter = 0;

void setup() {
  // serial ports are a way for your computer to send (TX line) and receive (RX line) data
  Serial.begin(9600); //initialize a serial port and set it to baud rate (speed) of 9600

}

void loop() {
  // put your main code here, to run repeatedly:
  Serial.print("counter:"); // print "counter"
  Serial.println(counter); //output counter
  delay(500); // delay 500 ms
  counter++; //add to counter

}