A candidate pawn is a pawn that has the potential to become a passed pawn once the opposing pawns on adjacent files are exchanged or eliminated.
In a given pawn structure, several pawns may stand on the queenside or kingside without an enemy pawn directly opposing them on neighboring files. Among these, the candidate pawn is the one most likely to advance freely to the eighth rank after a series of exchanges. For instance, if White has pawns on c4, d4, and e4 facing Black’s pawns on c5 and e5, the d4 pawn is the candidate: it has no enemy pawn on its own file, and once the c- and e-pawns trade off, the d-pawn’s path is clear.
In practice, identifying the candidate pawn in a position helps you define your long-term plan. If you own this pawn, your goal is to trigger the exchanges that will turn it into a passed pawn and then escort it toward promotion. If your opponent holds the candidate pawn, your priority is to blockade it — ideally by placing a knight directly in its path.