The pawn is the weakest piece and each player starts with 8 pawns. It represents infantry or foot soldiers. While there is noting in the official rules regarding what “pieces” are, the pawn is often ommitted when someone refers to their “pieces.”
Pawn movments are unique. It is the only piece that cannot move back/retreat. A pawn moves by advancing a single square, but the first time each pawn is moved from its initial position, it can advance two squares as long as the squares are not occupied or it is attempting to capture. The pawns method of capture is a diagonal move one square forward and one to the side, left or right.
Another uncommon move for the pawn alone is en passant capture. This is when a pawn uses its initial-move to advance two squares instead of one, and in so doing passes over a square occupied by an enemy pawn. That enemy pawn, which would have been able to capture the moving pawn had it advanced only one square, is entitled to capture the moving pawn “in passing” as if it had advanced only one square. The capturing pawn moves into the empty square over which the moving pawn moved, and the moving pawn is removed from the board. The option to capture en passant must be exercised on the move immediately following the double-square pawn advance, or it is lost for the rest of the match. The en passant move was added to the pawn’s repertoire in the late thirteenth century to compensate for the then newly added two-square initial move rule. Without en passant, a pawn could simply march past squares guarded by opposing pawns; en passant preserves the restrictive ability of pawns that have reached the fifth rank.
When a pawn reaches the opposite side of the board it is promoted to another piece of that player’s choice of a queen, rook, bishop, or knight. Promotion is often called “queening”, because the piece chosen is nearly always a queen. When some other piece is chosen it is known as “underpromotion” and the piece selected is most often a knight used to execute a checkmate or a fork giving the player a net increase in material compared to promoting to a queen.