Question as in the title. In this case, 83%. But they vary.
I guess this is the percentage of matched metadata compared between the existing metadata on the left-hand side of the lower Metadata Pane and the right-hand side of the same Pane.
A perfect metadata match (existing = new) would look like this:
You would also see a green tick in the top right-hand corner of the Album Pane for the same track:
