FEN is just a short way to describe the board-setup.
For black pieces lowercase-letters, for white uppercase, numbers
represent no pieces on sqaure.
So initial the fen notation would be rnbqkbnr/pppppppp/8/8/8/8/
PPPPPPPP/RNBQKBNR followed by a notation to keep track of castling
possibilities, who's move it is, the number of the move and which
halfmove it is. (I'm not quite sure of the sequence of the last part of
the FEN notation)