First, the code after "void setup() {" runs. This tells the Arduino that pin 13 is going to be sending data out. Most Arduino boards have an LED attached to pin 13. Then, the code after the "void loop () {" runs. When it reaches the bottom, the code after the "void loop" runs again, until the Arduino is turned off. This code makes the Arduino tell the LED to turn on, wait a second, then turn off and wait another second. Since it repeats, this code will turn the LED on and off again and again.