What's an unopposed Bishop? It's a Bishop where its counterpart on the opponent's side has been traded off. If you have two Bishops and your opponent has only a light-squared Bishop, then your dark-squared Bishop is unopposed. Similarly, if you have the last Bishop on the board, then it's also unopposed.
Unopposed Bishops are dangerous because they operate effectively against weaknesses on their color. An unopposed light-squared Bishop can attack Pawns on light squares or infiltrate the opponent's side of the board via undefended light squares.
The most dramatic example of the power of an unopposed Bishop is an endgame where each player has only Queen, Bishop, & Pawns, and the Bishops are of opposite color. If one player can attack the other's King without compromising his own King's safety, then that player has a real advantage. When both players are attacking the other's King, the variations become extremely complicated, difficult to calculate, and even more difficult to evaluate.
