Passed Pawns

A passed pawn has no enemy pawn on its own file or on either adjacent file in front of it, which means no pawn can ever stop it by capturing — only a piece can. That single fact makes passed pawns valuable, but it's a much smaller part of the story than most club players treat it as: whether a passed pawn actually wins a game depends far more on how easily it can be blockaded and how much material is tied down stopping it than on the simple fact that it's passed.

A piece parked in front of it neutralizes almost everything

A single well-placed piece sitting directly in the passed pawn's path — ideally a knight, since it doesn't block its own diagonal the way a bishop can end up doing — stops the pawn from ever advancing at all, regardless of how far it's already run or how few pawns remain to challenge it. This is the single most important practical fact about passed pawns: the pawn's danger comes from the threat of advancing, and a piece that removes that threat converts even a far-advanced passer into a pawn that isn't doing anything, as long as the blockading piece stays in place.

The tying-down effect matters even when the pawn never queens

A passed pawn doesn't have to promote to be valuable — a piece assigned to blockade it permanently is a piece that can't do anything else for the rest of the game, and that ongoing cost to the opponent is a real, ongoing asset for the side with the pawn even if the pawn itself never moves again. This is especially significant when the tied-down piece is the opponent's best piece; forcing a strong knight into permanent blockade duty in front of a passed pawn is functionally similar to trading it off, without actually giving up material.

Protected passed pawns are a different order of danger

A passed pawn defended by another pawn — a protected passer — is significantly more dangerous than an unprotected one, because a piece blockading it in front is still under attack from the defending pawn and can't simply sit there forever without support of its own. In king-and-pawn endgames specifically, a protected passed pawn is frequently winning almost by itself, since the defending king often can't approach the blockading square without walking into the pawn's own protector.

The passed pawn as a decoy, not just a promotion threat

In endgames with several pawns still on the board, a passed pawn on the far side of the board from the action — particularly an outside passed pawn, one on the a- or h-file with no other pawns anywhere nearby — can function as a decoy: the opponent's king has to spend several moves crossing the board to stop it, moves that the other side uses to attack pawns or infiltrate on the opposite wing while the enemy king is out of position. The pawn's promotion threat matters here mainly as leverage over the opponent's king, not as the plan itself.

When creating one costs more than it's worth

Not every trade that produces a passed pawn is automatically good. If the resulting pawn is easily blockaded, if the trade that created it weakens your position elsewhere, or if you don't have enough pieces left to actually support the pawn's advance once it's blockaded, forcing a passed pawn into existence can cost more in structural damage than the pawn itself is worth. Evaluate a potential passed pawn the same way you'd evaluate any other asset: by what it will realistically achieve, not by the label.

For the technique of stopping a passed pawn with the defending king specifically, see opposition, and for how a piece already restraining a pawn connects to broader restraint, restraint.