Doubled pawns
Doubled pawns are two friendly pawns stacked on the same file, which makes them less mobile and often easier to attack.
The front pawn blocks the back one. They cannot defend each other, and the file they sit on is usually half-open for the opponent’s rooks.
After the exchange on c6 in the Ruy Lopez, Black has pawns on c7 and c6. The extra c-pawn toward the center is not useless — it helps hold d5 — but the a- and c-pawns are a long-term structure White can play against.
Doubled pawns are acceptable when they open a file for your own rook or control key squares. They are a problem when they are isolated as well as doubled.