Making sense of this app's territory resolution
The score is B: because you're playing black, and it's showing your score.  
The black stone with the white circle on it is probably being scored as a dead stone right now - it's inside White's "territory".
Many apps do this if you try to score in the middle of the game - they estimate which parts of the board each player will end up with, and show a score based on that.  These estimates are usually not very accurate.


White gets some extra points called komi, because moving second is a disadvantage.  It looks like this game is probably giving White 7.5 komi.  I'll add this up using Chinese scoring, so you get one point for each point of territory you have surrounded, and each stone that is alive.  Black has nine points of territory and two living stones, for a total of 11.  White has 14 points of territory (the one under the "dead" black stone counts too) and three living stones, giving 17.  Once we add in 7.5 komi, black is behind by 13.5.