Black has just pushed the d-pawn two squares, from d7 to d5, straight past White’s pawn on e5. White answers exd6. The white pawn steps diagonally to d6 and the black pawn on d5 comes off the board, which makes this the only capture in chess where the capturing piece does not finish on the square of the piece it took.
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e f g h 1 e.p. is the label for that capture, short for en passant, French for “in passing”. It goes on the end: exd6 e.p. Some books print ep without the stops, older ones set it in italics, and computers do not write it at all.
The square you write is not the square you take
Here is where the notation trips people up. The move is exd6, not exd5. The laws leave no room for argument: the arrival square is the one on which the capturing pawn finally rests (Appendix C.8).
It makes sense once you see what the rule is pretending. En passant treats the black pawn as though it had only gone one square, to d6, and captures it there. The notation describes that pretend move, which is also the real journey your own pawn makes. Write exd5 instead and you have recorded a move that was not legal in the position, and anybody replaying the game from your sheet grinds to a halt.
The same convention shows up in FEN, the one-line format behind every diagram on this site. Its fourth field holds the en passant square, and that square is d6 as well: the one a capturing pawn would move to.
Do you have to write it?
No. The laws say it “may be appended” (C.8), and the PGN standard has no slot for it at all, so anything exported by a website or an app gives you a plain exd6. The label exists for human readers of printed games, where a pawn capture landing on an empty square looks like a misprint.
Write it if you like. I do, on a scoresheet, because it costs a second and it stops an arbiter frowning at the position later.
What the label does not do is buy you time. En passant is legal on the one move immediately after the two-square push and never again, whatever anybody writes down. The conditions in full are on the en passant rules, and if you would rather stop losing pawns to it than read about it, the en passant puzzles cure that faster.