int led1Pin = 13; 

void setup() {
  // pinMode() function set a certain pin to INPUT or OUTPUT
  pinMode(led1Pin, OUTPUT); 
}

void loop() {
  digitalWrite(led1Pin, HIGH); 
  delay(1000); // CPU pauses for 1000 millisecond (1 second)
  digitalWrite(led1Pin, LOW); 
  delay(1000); // CPU pauses for 1000 millisecond (1 second)
}