A passed pawn is a pawn with no opposing pawns to prevent it from advancing to the eighth rank, i.e. there are no opposing pawns in front of it, blockading it on the same file nor engaging it on an adjacent file (diagonally). A passed pawn is sometimes colloquially called a passer. Passed pawns can be an advantage because only the opponent's pieces can prevent them from promoting.
"Passed Pawns must be pushed"
"A passed pawn is a criminal that must be kept under lock and key. Mere police surveillance is not sufficient" -Aron Neimzovitsch
"Restrain. Blockade. Destroy." Nimzovich on passed pawns.