Pawns Can Advance Two Squares in the Initial Move
Pawn a2 may advance to a3 or to a4 in one move. That’s because, about four centuries ago, the rule was introduced stating that Pawns in their initial position, which are not blocked, may advance one or two steps.
The goal was to make the game more lively. Even though it wasn’t standard practice at first, the chess world accepted this rule over time.
With this rule, a difficulty arose. Although it could help accelerate the pace of the game during a chess tournament and add variety, the move sometimes betrayed the rights of the opponent’s pawn.
To illustrate this point, look at the two Pawns f2 and g4. The Pawn g4 stands on guard over f3. If f2 advances to f3, g4 can capture it; thus it has been for many centuries.
After the introduction of the new rule, Pawn f2 could evade Pawn g4 by advancing at once to f4 and could then molest Black’s e5 unpunished. Naturally, the Pawn g4 on guard feels deceived when the hostile Pawn creeps through the advance posts.
The Controversy and the Resolution: Capture in Passing
There were scenes of hot dispute; why should the new rule make the advancing Pawn immune?
After much controversy, justice was victorious. The Pawn standing on guard was given the right of capture. As such, Pawn g4 could still capture f2 while it tries to slip through just as if it moved one step only.
However, the Pawn on guard must execute this action immediately to prevent the attempted advance.
For instance, in the above position, white can move from f2 to f4. In response, Black can use Pawn g4 to capture the pawn “in passing,” just as if it had rested on f3. The capturing black pawn moves from g4 to f3, and the white pawn on f4 is removed from the board.
Thus, justice is served because Black executes their original intention of capturing the Pawn on f3. That’s why the move is named "capture in passing" or, with the French expression, capture "en passant".
See how Elliott explains capture en passant in the video below.