This is the Fast Track Access window. You can do fast track selections from here.
To play a particular track:
1. double click on the desired track in the tracks list
2. click on the corresponding track button
Click on the gizmo on the image below you want to learn more about:
Advanced Track Selection by number
When you need to select track with number greater than 20 you can use key [Shift]. When Shift downed the behavior of track buttons changes and you can simply enter the desired track number with keyboard or mouse:
[Shift down] -> [Entering track number] -> [Shift up]
See also: