To compare one piece to another, place one on an empty board and count the squares it can move to.
From a central square, bishops hit 13 squares while knights hit 8. But the knight makes up some value by being able to move to both colors, while the bishop is locked on the same color.
The relative power of knights and bishops changes greatly depending on the friendly pawns on the board. Bishops much decrease in value when friendly pawns are on the same color square as the bishop because that limits the bishop’s mobility.
The more open the board, the more of an advantage the bishop has over the knight. In a blocked position, the knight might take the lead. So that aids in one’s planning. When you have bishops, open lines for them. When you have knights, keep the position closed.
In general, a bishop is worth a bit more than a knight. If a player can get a bishop in exchange for a knight, and other positional factors stay equal, s/he has done well.