Passed Pawns
A pawn is passed when there is no enemy pawn anywhere on its own file or on either neighboring file, ahead of it. That means no enemy pawn can ever block its path or capture it as it marches forward -- the only things that can stop a passed pawn are enemy pieces (which cost the opponent time and coordination to bring back to stop it).
Passed pawns are one of the most important assets in chess, especially in the endgame. Tarrasch's advice to "always put a rook behind a passed pawn" (your own to push it, or the opponent's to blockade it) exists precisely because passed pawns are so dangerous: a passed pawn on the 6th or 7th rank can force the opponent's pieces into permanently passive, defensive roles just to stop it queening.
Not all passed pawns are equally strong -- a passed pawn deep in enemy territory, supported by pieces, is far more dangerous than one still on its starting square. But the defining property is always the same: no pawn, ever, can get in its way.
Worked examples
Three passed pawns
With no black pawns left on the board at all, a5, c4, and d2 (the most advanced pawn on each file) all qualify as passed.
A full rank of passed pawns
Every one of White's eight pawns is passed here, simply because Black has no pawns anywhere on the board to ever contest them.
Test yourself
Click all three passed pawns.
0 of 3 found