Doubled Pawns

Pawns get doubled when one pawn captures onto a file that already has a friendly pawn on it (or, less commonly, through promotion). The result is two pawns sharing a single file, one in front of the other. Because pawns can only capture diagonally, the rear pawn can never simply walk past its own pawn in front, and the two of them together only really control the squares one pawn on that file would -- they've lost a lot of their combined value without losing a pawn's worth of material.

Doubled pawns are usually a structural weakness for two reasons: they can't defend each other (defense is diagonal, not vertical), and a file with doubled pawns is nearly always missing pawn coverage on at least one neighboring file too, which tends to make the doubled pawns isolated as well as doubled -- a double weakness in one package.

They aren't purely bad, though. Doubled pawns can open a file for a rook, add extra control over key central squares, or come from a trade that wins the bishop pair or damages the opponent's structure even more. Whether doubled pawns are a real problem or an acceptable price depends heavily on what the rest of the position looks like.