FEN is a code for the position. When you have a certain situation at the board there is one FEN describing the situation. Computers know what the position is when you give them a FEN code.
For more information:
http://www.chess.com/forum/view/general/fen-and-pgn2?lc=1
FEN
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On the screen for playing turn by turn chess, on the right hand side below the notation screen, there is a box with "FEN".
Where can I find the meaning of FEN.
Thanks