A harpejji is a neat stringed instrument that has frets like a guitar but is finger tapped. The strings are tuned in whole steps so it is possible to quickly transpose a melody to a new key by maintaining its "shape". C notes are designated with a dot.
This app isn't meant to substitute the far superior real harpejjis but to give users a chance to learn and experiment with their unique chromatic layout.
Choose landscape (better) or portrait mode and number of frets (I prefer 6-8).
You can learn more about real harpejjis at https://www.marcodi.com
Some tips on how to play: https://www.marcodi.com/pages/learn-to-play