In chess, doubled pawns are two pawns of the same color residing on the same file. The only way such pawns can arise are by pawns making a capture. In the diagram, the pawns on the b and e file are doubled. The pawns on the e file are doubled and isolated. (Also see isolated pawn and backward pawn.)
In most cases, doubled pawns are considered a weakness, especially when they are also isolated. This is because doubled pawns are unable to defend eachother and because in the endgame such pawns are worse at achieving a breakthrough which could create a passed pawn. Several chess strategies and openings are based on burdening the opponent with doubled pawns and a strategic weakness. However, there are cases where accepting a doubled pawn can be advantageous because doing so may open up lines for a rook, or because the doubled pawns perform a useful function such as attacking important squares. Also, if the opponent is unable to effectively attack the pawns, their inherent weakness may be of little or no consequence.