The MIDI Output Window (also known as Real-time Display Window) shows what's happening on each channel -- what instrument is selected at that moment for each channel specified in the MIDI file playing.
You can mark a channel (that is, instrument) as a solo instrument by clicking the blue button (BLUE=SOLO). You will still hear the other instruments faintly. You can mute one or several channels by clicking the red button (RED=STOP). If you want to save these changes to the playlist so that they will be remembered in the future, select the control window and click on the Save-to-list button.
In order to save the settings from the Output Window permanently in
the playlist, use the Control
Window.