Good vs. Bad Bishop:

A Bishop is considered good when it's own pawns are on opposite color squares while the enemy pawns are on the same colored squares. When this happens the Bishop is not blocked by it's own pawns while simultaneously being able to attack the enemy pawns. Similarly, a Bishop is considered bad when it is blocked by its own pawns being on the same square and is also unable to target the enemy pawns since they lie on opposite colored squares. In the example below, White has the good Bishop while Black has the bad Bishop.