When you're able to create (and use) strong and weak squares, they may help you win the game.
So, what are weak squares?
Weak squares are squares which can't be defended by pawns.
In this diagram you can already spot some weak squares. In Black's camp you'll discover a6, c6 and d6 as being weak squares. There are no pawns left to defend these squares. With pawns at b7 and c7, these squares weren't weak.
You may have noticed that all of the squares behind the pawns are weak by default.
Can you identify White's weak squares?
This diagram may clarify things even further. Now a3 and c3 have become really strong squares for Black. Not only are they weak from White's point of view (not defendable by a pawn), Black has taken control over these squares with his pawns.
If Black would be able to land a piece there, the piece would be supported by the pawn at b4.
You may have noticed that there are some strong squares for White as well. The a4 and c4-square are excellent squares for white pieces, as they are controlled by the white pawns and no black pawns can chase the pieces away.
There's one very important difference between the black and the white strong squares. Black has his strong squares on the white side of the board. If he would land a piece there, it would stand in the heart of White's position. This would make White's life very difficult.
Black's weak squares are much less threatening. If White would land a piece there, the piece would still be on the White half of the chessboard. This would be a lot less intimidating.
In many positions, both sides have strong and weak squares. The side that makes the best use of his strong squares is most likely to win.
This pawn structure, arose in a game where Paul Keres played Black.
When you identify the weak and strong squares, you'll notice that White may use the strong square f5, whereas Black has control over the d4-square.
Let's take a look at the end of this game. I'll first put the pieces in.
Now you may notice some significant differences. White will be able to make use of his strong f5-square almost immediately. Black has a much harder time to get his knight to d4.
White is ready to use the hole on f5 for his knight.
Paul Keres tries to give his black knight a future, but weakens his position further. Now the c6 and e6-square have become weak and his d-pawn may become a target.
Note that c5 isn't a strong square for Black, as a white pawn on b4 would control c5 as well.
2.Nf5+, Kg6; 3.Qc3,
The knight is very well posted on the strong f5-square. This gives white excellent control over the position.
Black has defended the h5-pawn (and the kingside in general), so White switches to the other side.
Black is already much worse here. His knight isn't able to reach the d4-square and White will be able to win the d6-pawn whenever he wants. I'll give you the rest of the game as an example on how to use weak / strong squares.
3....Na6; 4. Qc6, Nc5; 5. f3,
White isn't hasty. He can capture the d-pawn any time he likes. First he defends the e-pawn.
Do you notice the unfavorable position of the black knight? It has no influence in white's camp, whereas the White knight has excellent influence on black's position.
5. ...Nd3; 6. Qc7,
White doesn't capture the d-pawn as this would give Black the chance to play either h4 and take control of the f4-square (when the knight captures) or (when the queen captures) Nc1 and win one of the queenside pawns.
Now Black has no useful moves, as the queen must defend the g7-square against mate.
This ended the game. The black queen is still paralyzed, and the black knight doesn't achieve anything. The threat of Nh4+ and Qe3+ is already there, so Keres decided to call it a day.