Simple Arduino Blink example demonstrating the use of a variable to store the pin number for the LED

blink.ino

int ledPin=13; //This is a variable to store the pin number that we want the LED on
 
// the setup function runs once when you press reset or power the board
void setup() {
  // initialize digital pin as an output.
  pinMode(ledPin, OUTPUT);
}
 
// the loop function runs over and over again forever
void loop() {
  digitalWrite(ledPin, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(1000);              // wait for a second
  digitalWrite(ledPin, LOW);    // turn the LED off by making the voltage LOW
  delay(1000);              // wait for a second
}