Blink, Line by Line

What you'll learn

• Every Arduino program has two boxes: setup() runs ONCE when the board powers on, loop() runs again and again forever (with nothing to slow it down, thousands of times a second).
• Curly brackets { } are the walls of each box. void = the box hands nothing back. // starts a note for humans — the chip skips it.
• pinMode(LED_BUILTIN, OUTPUT); — LED_BUILTIN is the name for pin 13 (the little "L" light on the Uno). OUTPUT = the pin pushes power out to drive something; INPUT = it listens.
• The semicolon ; ends every instruction, like a full stop. Forget it → the upload stops with "expected ';'". Leave out pinMode → the pin stays an input and the light glows only very dim.
• digitalWrite(LED_BUILTIN, HIGH); = 5 V out, light on. delay(1000); = wait 1000 milliseconds = 1 second.

  digitalWrite(LED_BUILTIN, LOW); = 0 V, light off. Then loop() starts again from the top.

• Capital letters matter: digitalwrite (all small) is an error.
• Delete the last delay and the light looks always on — it's off for only a few millionths of a second.