First, the initial -6,5. That is komi. In an even match, white is given 6,5 point from the start, to compensate for black playing first (which is an advantage of course). Thus in the moment the match begins black is losing by 6,5 because neither player made any points yet. The half point is to prevent draws.
At the end black is losing by that many because he made no territory. (while white made plenty) All the black stones (even the ones still on the board) are dead, because they cannot make two eyes now to live. Thus black is losing by that much.
In go (let’s talk japanese rules, but it does not really matter) you get points for capturing stones and above all surrounding territory. Black surrounded no territory and thus made no points.
Maybe you were thrown off by the minus. The app counts the score towards the player thus if you were winning it would still read the score for black, but there would not be the minus before the score