F.E.C. (Frequently Encountered Confusions):
Brown line is the Volume Envelope.
It's similar to an ADSR envelope, except you draw the whole thing by hand instead of using numeric values.
Green line is the Pitch bend (frequency shifting).
Center of the drawing area is the frequency you pick. Top and bottom edges are the octave above and octave below that, respectively.
The string animations are for visualization only.
They serve no function. Except visualization.
The letters on the piano keys mean you can type to play notes.
You can click the grey boxes below the piano keys to move the keyboard bindings around.