setup is run once, when the program first starts running.
loop is run over and over, forever.
digitalWrite writes to one of the Arduino's pins.
delay waits that many milliseconds (thousandths of a second)
before continuing the program.
Now try to make it blink faster or slower -- how would you do that?
Now add a second LED, and make it the opposite of the first one: when your red LED is on, the green one is off, and vice versa. (Hint: plug the second LED into a different pin. How about adding a second LED? Can you make them blink together? (Hint: you can use any of the Arduino's digital pins, not just 13.)
How about making one LED be ON while the other is OFF?
If you get something that works, you can save it, using
Give it a name describing what it does, like 2led or one-on-one-off.
Any time you change a sketch to make it do something new, you can save
it to a new name -- that way you can go back to it later.
Too easy? Try a problem that's a little harder: make your two LEDs blink at different rates. For instance, make the green one blink twice as fast as the red one.