When a pawn reaches the opposite side of the board, it promotes (often to a queen but potentially to a rook, bishop or knight too). In the endgame this is a very common idea. Here, white has equal material but would be struggling without a tactical idea.35c7!...Advancing the pawn and creating the threat of promotion! The promotion square is also defended by the knight, so a move such as Qc5 to cover the promotion square doesn't help.35...Qxd635...Qxc7?Qxc7 would be a mistake, due to the unfortunate placement of the black king, white could fork the royal pieces.36.Ne8+36c8=Q...Since black cannot capture the pawn, the best available is to take the knight instead, but now white is in a much better position with a new queen and more dangerous pawns threatening promotion.