Each piece has a numerical value that can help you evaluate whether exchanging it makes sense.
Pawns = 1.
Knights = 3.
Bishops = 3.
Rooks = 5.
Queens = 9.
Kings = The whole game!
This tells you that trading a knight for a bishop is OK, but you wouldn't want to lose a queen and only get a rook. When evaluating possible trades you should try to capture as much or more value than your opponent does unless you have a very good reason.
