FEN is a way of recording positions, not moves.
The gist is that, looking at the chessboard from white's point-of-view, you record the pieces on the board from the furthest row to the nearest, left to right. A quantity of empty squares are represented by a number. 3 empty squares = "3"
Information for the rows is separated by a "/"
For example, if black had rooks on a8 and h8 and a king on e7, those two rows of FEN would be: r6r/4k3/ and would be interpreted as:
Farthest rank. rook, 6 empty spaces, rook. One rank closer. 4 empty spaces. King. Three empty spaces. One rank closer...
Finally, after the row information, FEN shows "state" information. That is, can white and black castle kingside and queenside? Is an en-passant capture possible? Who's move is it?
A better desription can be had my Googling and selecting the link to wikipedia.
Could someone please explain what the FEN is? I know that it's a way of tracking the moves that have been made during a game, but what do all the different letters mean? How do I figure out how to interpret it?