The download button opens the iTunes App Store, where you
may continue the download process. You must have iTunes installed with an
active iTunes account in order to download and install the software. This
download may not be available in some countries.
From Double Slash Design:
Music lessons ( tab notation ) for experienced players, Guitar, Bass, Ukulele. In-App Purchases will consisit of additional lessons for the users. Additional Instruments and lessons will be added in future updates