"Miss" stands for a "Missed Opportunity." Your move isn't bad. We can see that the engine likes your move because it evaluates it at -1.33 advantage for black; it's just that it was even more of an advantage after bxc5 which the engine evaluates as -2.59 advantage for you. A computer can't "think" (at least not in the way humans do) and so it determines the game report based on evaluating what it considers to be the optimal move in the position; it calls that move "best" and then evaluates all other moves by how much "worse" it is compared to that move. A "blunder" is worse than a "mistake" and there's very technical centipawn differences and criterion among all these descriptions; long story short is that your move can still be good even though it's not the "best" move.
As for the "why" behind this move, it has to do with the fact that bxc5 opens up the b-file for black's rooks (and possibly queen) to use for attacking the white king castled on the queenside. This is one example position where doubled pawns are desired and there's other examples too 
I'm going through this game in Analysis and it's showing me Bxc5 as a miss, but bxc5 is best. Why is this?