The Harmony/Melody switch
This applies only when displaying the "short staff" edit window that is used for exercises involving single pitches, single intervals, and single chords.
When available this toggles between "harmony keys" and "melody keys." "Harmony keys" is also known as "sticky keys" - when you play a keyboard note with the mouse it will stick on visually (the sound will stop when you release the mouse). In harmony mode you'll click that same note again to turn it off. The reason for "sticky keys" or "harmony mode" is that without it you wouldn't be able to play a chord using only the mouse.
With the "melody" setting chosen the keyboard acts like a normal piano: notes turn off when you release them and also disappear from the staff.