Promotion
A pawn that reaches the last rank must immediately become a queen, rook, bishop or knight of its own colour. The choice is free and unlimited — you may promote to a piece even if one of the same kind is already on the board. Underpromotion to a knight is a genuine attacking tool when a queen would give stalemate or miss a fork.
a8=Q: the pawn leaves the board and the new queen appears on the promotion square itself.