These part tracks can be further subdivided into 'channel tracks' corresponding to the different sounds used within the staff. MuseScore creates one 'part track' for each staff in your score. Clicking on one of the panels selects it, and also allows you to edit other instrument parameters, such as choice of instrument, name, pan, chorus, reverb, etc.
The Track Area displays allows you to adjust the volume of the instruments used by your staves. The track area is at the bottom and displays the Master Gain as well as a row of tracks. The Mixer is split into a Details Area on the top and a Track Area below. To display/hide the mixer, use one of the following: The Mixer allows you to change instrument sounds, adjust volume and panning, and make other adjustments to the playback for each staff.