Black has pawns on c6 and c7, one behind the other, and no way to fix it. They arrived by capture, which is the only way doubled pawns ever arrive: 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Bxc6 dxc6, the Exchange Variation of the Ruy Lopez.
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e h 1 What the doubling actually costs
Three things. Two pawns on one file cannot defend each other, so both need pieces to look after them. They are slow, because the back one can only move when the front one does. And the file next door is now empty, which is why doubled pawns and half-open files always turn up together.
The one that decides games is arithmetic. Play the line under the diagram to the end and count: Black has four queenside pawns against White’s three, which is normally enough to produce a passed pawn. Not here. Advance them and White’s two hold all four, because two of Black’s are stacked on the c-file. White’s four against three on the kingside does make a passer. That is why the Exchange Ruy exists at all, and why White swaps queens on move eight and walks into the endgame.
What they are worth in return
Look at what Black got. The capture on c6 opened the d-file for a rook and left Black with two bishops against a bishop and a knight, which in an open position is a genuine advantage worth having. The pawn on c6 also covers d5 and b5, two squares White would like.
So the honest answer to whether doubled pawns are bad is: they are a long-term structural debt, and the payment you took for them decides whether it was a good deal. Keep queens and bishops on and Black is comfortable. Trade everything and White is grinding.
The version that really is bad
Doubled and isolated at the same time. Pawns on c6 and c7 with no b-pawn and no d-pawn anywhere is the worst pawn shape in chess: neither can be defended by a pawn, they get in each other’s way, and both sit on a road that enemy rooks own. That is the same weakness as an isolated pawn, doubled.
The place to see all of this decided is the pawn endgame, where a majority that cannot make a passed pawn is simply a majority that does not count.