The terms "good" or "bad" bishop refer primarily to the squares that the central pawns are on. It is possible for a "bad" bishop to be well placed and useful, or vice versa.
In this case, white's central pawns control the dark squares, so his light squared bishop is a good bishop, and his dark squared bishop is a bad bishop. Because of the position, even his bad bishop is well placed, though. Because of the black pawn on e6, the black light squared bishop is technically a bad bishop, though the fianchetto position could give it pretty good freedom of motion here. But black doesn't have any pawns on dark squares in the center in this position, so his dark square bishop could have been used to fight for control of those squares. That's why it was his good bishop, and trading it off was probably a mistake.
--Fromper
While reading a chess problem, part of the caption for it was "Black has played quite carelessly and traded off his important dark-squared bishop." What makes a bishop important in the opening? Does one important bishop mean that the other isn't important? If it helps you understand the context, here is the position:
I'm assuming that black recently traded his dark squared bishop for white's queen-knight at c3.