Accordelli represents part of the left-hand bass keyboard on an accordion. Often used for accompanying tunes and especially noted for providing an "oom pah pah" rhythm - ie C, CM, CM.
These notes are usually played short; on a real accordion the buttons are stabbed at rather than held. The keyboard is laid out with 2 rows of notes and 2 rows of chords (major and minor).
Adjacent notes go up in fifths and so provide for easy chord progressions. One difference with Accordelli is that an extra C is provided at an octave higher than the one in the middle.
This is so that a complete octave can be played ( in the key of C) on the note buttons - it is labelled "Co".
On the flipside of the main screen you have a little control over the length of chords and notes.