All the details are available on a GitHub page, and it’s a modified version of an Arduino guitar tuner he found on Instructables ... which you can see in the very short YouTube clip below.

Everything starts with tuning; and if you're a guitarist it's a burden you have to accept. Pianists? They have it easy! Set and forget. But every time you pick your guitar up to play you'll need ...

Both the App Store and Google Play are replete with guitar-tuning apps that are full of either upsells or advertisements. Isn't there any application that helps tune your guitar without bothering you?