What is FEN format? How to use it? For what it is used?

I believe to store a position instead of a game

It describes a board, including details about castling, en passant, # half-moves, and so on.

See Wikipedia for more info.

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 an integral part of the Portable Game Notation for chess games, since FEN is used to define initial positions other than the standard one.

 

Wikipedia 

Thanks.

and how to open a .pgn file?

You can open .pgn files with Notepad if you like.  It is just a plain text file that records a game.  Here's an example.

---

[Event "F/S Return Match"]
[Site "Belgrade, Serbia JUG"]
[Date "1992.11.04"]
[Round "29"]
[White "Fischer, Robert J."]
[Black "Spassky, Boris V."]
[Result "1/2-1/2"]
 
1.e4 e5 2.Nf3 Nc6 3.Bb5 {This opening is called the Ruy Lopez.} 3...a6
4.Ba4 Nf6 5.O-O Be7 6.Re1 b5 7.Bb3 d6 8.c3 O-O 9.h3 Nb8  10.d4 Nbd7
11.c4 c6 12.cxb5 axb5 13.Nc3 Bb7 14.Bg5 b4 15.Nb1 h6 16.Bh4 c5 17.dxe5
Nxe4 18.Bxe7 Qxe7 19.exd6 Qf6 20.Nbd2 Nxd6 21.Nc4 Nxc4 22.Bxc4 Nb6
23.Ne5 Rae8 24.Bxf7+ Rxf7 25.Nxf7 Rxe1+ 26.Qxe1 Kxf7 27.Qe3 Qg5 28.Qxg5
hxg5 29.b3 Ke6 30.a3 Kd6 31.axb4 cxb4 32.Ra5 Nd5 33.f3 Bc8 34.Kf2 Bf5
35.Ra7 g6 36.Ra6+ Kc5 37.Ke1 Nf4 38.g3 Nxh3 39.Kd2 Kb5 40.Rd6 Kc5 41.Ra6
Nf2 42.g4 Bd3 43.Re6 1/2-1/2 

---

 

Many chess programs will also open .pgn files so you can playback the game. WinBoard being one of the nicer free options.