A person who is said to be a pawn is being manipulated in an expendable manner. It's a negative connotation typically directed at someone who blindly follows, and to use another phrase, "falls in line" against this person's own interests.
Arguably, this can be said of all the pieces in chess, as they are all being used by an all-seeing player who is playing a game with these pieces' livelihoods, however, it's more effective to call this person a pawn, being the lowest valued piece on the board.