It means they were just a person pushed out into battle by more high ranking persons.
"He was just a pawn." What does this mean?

It means that the person (usually an enemy) is simply following the orders of someone else above him - in other words, he is not the enemy leader that the protagonist is looking for.

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.
I have heard the phrase - "He/She was just a pawn." many times in movies and television. So what is that supposed to mean.
Also has anyone watched 'Endgame' TV series?
http://www.imdb.com/title/tt1797629/
Trailer: http://www.youtube.com/watch?v=xCtaUpLx6QA