Connected pawns can defend each other and support each other as they advance. Doubled pawns block each other from advancing. Pawns, with no other pawns next to them are isolated, and are frequently easy to attack.
In this position black has pawns on a6 and c6, but they are too far apart, they can't protect each other.
These pawns are called isolated pawns and can easily become a target, in this position just by moving the King you threat taking one of them.
But you can also move the bishop to c4 and target the a6 pawn, either way, neither pawn can be protected and both will fall soon.
