Strength is life. Weakness is death - Swami Vivekananda

Chess mastery is all about perception of weakness - as one rises in their level of mastery, what gets attuned and heightened is their perception of weakness: from gross to finer and at supreme level of mastery, the weakness would appear almost insignificant!

Even amongst great masters, one can find occasionally that one succumbs from myopic tendencies when perceiving weakness and therefore great masters too find themselves on the wrong side.

Let us get down to basics first.

What is weakness in chess? Let us list them out.

• The King that has no minor pieces around
• The King whose pawn structure is breached, or the pawn has moved in front of it.
• Loose piece which are hanging around without any support
• The open lines (diagonals or files or ranks) that fall into the hands of the opponent, as none of your pieces control them or can counter them

We can keep adding to this list generically and also refine them on the go, during the course of a game, according to the type of position that arises.

But primarily, the weakness is all about the Squares! In Chess, as one advances in their level of mastery, the perception of weakness gets finetuned and it relates only to squares! And a piece or pawn that happen to be on that square become weak consequentially! 

This can happen momentarily due to a weak move by a player, or it may happen over a period of time when a particular move which creates a backward pawn or a weak square was not considered as weakness because at that time, the concerned player may have had adequate pieces to defend them or to ward off the opponent's pieces from occupying the weak squares. 

For example:

                                                                

Abramovic. Bosko - Damljanovic, Branko - Novi Sad, 1995

In this position, it is apparent that the "e6" square is very weak and White has posted a very powerful Rook on that square which exerts pressure over the adjacent squares, say d6 and f6 and consequentially the other squares around that area also gets exposed. The weakness of a particular square generally will not be localised to that square but may spread over a broader spectrum.