Here's some information that I cut and pasted from online when I googled "FEN".
Forsyth–Edwards Notation (FEN) is a standard notation for describing a particular board position of a chess game. The purpose of FEN is to provide all the necessary information to restart a game from a particular position.
FEN is based on a system developed by the Scottish newspaper journalist, David Forsyth. Forsyth's system became popular in the 19th century; Steven J. Edwards extended it to support use by computers. FEN is an integral part of the Portable Game Notation for chess games, since FEN is used to define initial positions other than the standard one. FEN does not represent sufficient information to decide whether a draw by threefold repetition may be legally claimed or a draw offer may be accepted; for that, a different format such as Extended Position Description is needed.
A FEN "record" defines a particular game position, all in one text line and using only the ASCII character set. A text file with only FEN data records should have the file extension ".fen"
A FEN record contains six fields. The separator between the fields is a space. The fields are:
PIECE PLACEMENT (from white's perspective):
Each rank is described, starting with rank 8 and ending with rank 1; within each rank, the contents of each square are described from file "a" through file "h".
Following the Standard Algebraic Notation (SAN), each piece is identified by a single letter taken from the standard English name.
pawn = "P", knight = "N", bishop = "B", rook = "R", queen = "Q" and king = "K".
White pieces are designated using upper case letters (PNBRQK) while black pieces use lower case (pnbrqk).
Empty squares are noted using digits 1 through 8 (the number of empty squares),
and the "/" separates ranks within the placement field.
"w" means White moves next, "b" means it is Blacks turn to move.
If neither side can castle, this is "-". Otherwise, this has one or more letters: "K" (White can castle kingside), "Q" (White can castle queenside), "k" (Black can castle kingside), and/or "q" (Black can castle queenside).
EN PASSANT TARGET SQUARE (in algebraic notation):
If there's no en passant target square, this is "-". If a pawn has just made a two-square move, this is the position "behind" the pawn. This is recorded regardless of whether there is a pawn in position to make an en passant capture.
This is the number of halfmoves since the last capture or pawn advance. This is used to determine if a draw can be claimed under the fifty-move rule.
The number of the full move. It starts at 1, and is incremented after Black's move.
A Few Examples
Here is the FEN for the starting position:
rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
Here is the FEN after the move 1. e4:
rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1
And then after 1. ... c5:
rnbqkbnr/pp1ppppp/8/2p5/4P3/8/PPPP1PPP/RNBQKBNR w KQkq c6 0 2
And then after 2. Nf3:
rnbqkbnr/pp1ppppp/8/2p5/4P3/5N2/PPPP1PPP/RNBQKB1R b KQkq - 1 2
I hope this helps.
You can enter "chess fen description" into a search engine to get more details or examples.