A protected passed pawn is a passed pawn (meaning no enemy pawn can stop it on its file or on adjacent files) that is defended by another friendly pawn. It is one of the most powerful structures in chess, combining two strengths: the unstoppable nature of a passed pawn and the solidity of pawn protection.
Picture a white pawn on d5 supported by a pawn on c4 or e4, with no black pawn able to challenge it. This pawn marches relentlessly toward promotion, forcing the opponent to tie down an entire piece — often a rook or bishop — to watch it permanently. That piece becomes passive and loses its ability to contribute elsewhere on the board.
In practice, a protected passed pawn in the endgame is frequently decisive. The plan is straightforward: simplify the position by trading pieces, since in a pure pawn ending the protected passed pawn wins automatically. The fewer pieces remain on the board, the greater its power becomes.