Look up the en-passant rule. Its a valid move.
Steve
It's a pawn rule that doesn't get used in the majority of games played, "En Passant." Essentially, in the above position, if White has a pawn on the 5th Rank (in this case the White pawn is on c5) and either the b or d pawn moves two squares in order to sneak past the White c pawn, White can capture "En Passant." Look up the rule in any chess book and you'll see how it works. It probably caught you off guard because it isn't used as much in the majority of games played.
like how does this happen?