The pieces have different values; the kingK cannot be measured but is the highest and most important piece.  

The second-highest piece is the queenQ.It has the value of 9 pawn units. To loose it means the biggest material disadvantage that will mostly lead to loss of the game.  

The queen is followed by the rooksRR.They are measured with 5 pawn units. Queen and rooks are called "heavy pieces". 

The bishopsLLand the knights NNhave the same value of 3pawns. They are called"light pieces".

Then at last the pawnsPPPPPPPP.A pawn  has naturally the value of one pawn, therefore the pieces are measured in pawn units.