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, white has three pawns side by side.
If black wants to attack any pawn, it has to move a long way.
If black wants to attack the a2 pawn moving the bishop to e6, white can respond with the simple move a3 and the black king cannot take the pawn.
