Understanding the value of pieces!
Know the point system
Bishop vs Knight (both are equally worth 3 pieces)
In some situations, such as an open position, a bishop may be stronger than a knight.
In some situations, such as a closed position, a knight may be stronger than a bishop.
Rook vs 2 minor pieces (2 bishops, a bishop and a knight, or 2 knights)
In many case, it is worth trading your rook (and a pawn) for 2 minor pieces!
While both are worth 6 points, having 2 minor pieces to one major piece (rook) can lead to a HUGE advantage if you use it right.
Do NOT be worried about trading queens!
A general rule of thumb, when you are winning, it is a good idea to trade pieces to simply the board. When you are losing, you want to avoid trading pieces!