The Chord Matrix appears in Track View when the Chords track has the focus.  It is accompanied by the relations panel and the chord bar.

It allows you to select the chord in the current segment.  If a Scales track is also included in the song, there may be red dots indicating which chords are relations of the scale at the focus location.

When you click on a cell in the chord matrix, the sound is auditioned.  You can also audition any cell without selecting it, by using the middle mouse button, or the left mouse button while the Ctrl key is pressed.

The chord matrix has several methods you can use for navigation and selection.