As the name suggests pawn promotion is a reward for the pawn only. The pawn will get this reward if it is able to march throughout the board, through the enemy’s territory and get to the other side of the board. The reward for the pawn in this situation here is that when it reaches to the other side of the board successfully he now has a choice to turn itself to a stronger piece, either a knight, a bishop, a rook or a queen. The pawn cannot remain a pawn and it cannot turn to the king because there can only be one leader.
Now very rarely you will change into a rook or a bishop, mostly the pawn turns to a queen because the queen is the strongest of all pieces. However, if you change to a bishop, a knight or a rook it is called as under promotion. One thing to be noted here is that even if you already have a queen on your board does that mean that you cannot change your pawn to another queen? No that is not true, you can actually change the pawn to a second queen, and it won’t matter. That is why an additional queen is given with the chess set.