Whether you're working with the Raspberry Pi, an Arduino or a Beaglebone, a lot of the time you'll want to load up programs automatically on boot. There are a variety of ways to do so, but Adafruit guides you through the best practices for each device.
On Arduino, you'll use software called a bootloader, while with Linux systems it's preferable to use the init system. There are other options, and Adafruit breaks them each down with pros and cons of using each. It's useful, because if you're making any type of single-purpose electronics project, you're almost always going to want to launch on boot.