What does this mean?

7. cxd4

I don't understand what the c means in the notation? I know it seems stupid, but I am new to the game. Can someone tell me what the c means? 

The pawns are "a"through "h".  So, the "c" pawn.

Yes, it means that the pawn on c takes on d4. When a piece takes on a square, for example, a knight takes on d4, we write Nxd4. And when there are two knights on different vertical files that can take this pawn, we write, for example, Ncxd4, to avoid ambiguity.

So, Ncxd4 means "knight on the c-file takes on d4". For a pawn, similarly, you could write Pcxd4, meaning "pawn on the c-file takes on d4". But we don't use "P" for pawn, we assume that, if the piece isn't specified, it is a pawn. So we write cxd4.

Compare the notations for different moves: