A chess piece of the lowest value that can move one square forward at a time, with an optional first move of two squares. It can take other pieces by moving diagonally and can be exchanged for any other captured piece on reaching the farthest rank of the board. Which piece I'm talking about..?

The Pawn is correctly identified as the piece described. Pawns move one square forward (with an optional two-square first move), capture diagonally, and can be promoted to any piece except a King when reaching the eighth rank. Rooks, Queens, and Kings have completely different movement patterns and are much higher in value.