The distinction between good and bad bishops depends on the pawn structure. If most of your pawns are on light squares, your dark-squared bishop is the "good" one because it can move freely without being blocked by your own pawns. Conversely, your light-squared bishop would be "bad" because its diagonals are clogged.
A good bishop becomes especially powerful in open positions where long diagonals are available. In endgames, a good bishop can be worth significantly more than a bad bishop and may even outperform a knight when the position is open and the bishop's range gives it a decisive advantage.
To maximize a bishop's potential, structure your pawns on the opposite color from your remaining bishop. If you have a light-squared bishop, try to place your pawns on dark squares. This principle should influence both your opening choices and middlegame pawn play. Trading a good bishop for a knight is generally unfavorable unless there is a specific tactical or strategic reason.