Pawns may be the weakest pieces on the board, but they have the potential to become much stronger. Should a pawn manage to make it all the way to the other end of the board, that pawn must promote
to any piece its owner wants, other than a king. Most of the time, a pawn will be promoted to a queen; however, you can also choose to promote to a rook, knight or bishop. Promoting to something other than a queen is known as underpromotion
The diagram above shows a pawn from each side preparing to promote.