Clicking on the piano keys sends midi note data to the selected MIDI output port (see MIDI Setup).

1-16 buttons set the MIDI Channel for the out going MIDI notes

Adjusting Velocity slider (on the right side of the window changes the out going MIDI note's Velocity

Clicking on the Octave arrows shifts the keyboard's octave setting. (Q,W,E,R,T,Y,U,I,O,P computer keys also do the same trick)