The Harp is a musical item which is typically used for puzzles in Adventure Mode. Right-click to open the item screen. Use the 0-9 keys to play notes, and hold Shift to play sharps. Playing this as well as the Guitar and the Snare Drum can be used to trigger scripts.
Harp Hotbar Icon
To create scripts, create a file called “MusicScripts.txt” in your map folder. In the file, create a new line for each song as follows: [string of numbers 0-9], [script filename], [name of song]. Once the string of numbers is played by an instrument, the chosen script will run.