1. b6 is weak
2. a5 is weak, but also a7 and b7
A pawn or a square in a player's position that can be either effectively attacked or pressured. A weakness can be like a bleeding ulcer draining a player's resources, leaving them unable to concentrate on anything but its defence. Generally, however, two weaknesses, usually multiple files apart, need to be inflicted on an opponent in order to secure victory.
In position 1, b6 is attacked by four white pieces. Consequently, black is completely tied down to defending it.
In position 2 (Kasparov - X3D Fritz, 2003), black's a5 pawn is weak, and will eventually fall. And when it does, the a7 square will become vulnerable to invasion and the b7 pawn to attack.
Every pawn move leaves behind it potential weak squares or creates potential pawn weaknesses.