White's pieces can move to c5 and f5 without the slightest worry that they will be attacked by a Black Pawn. Both of these squares would be ideal for a White Knight or a White Bishop.
The corresponding squares on the White side of the board -- c4 and f4 -- represent no particular danger for White. If a Black piece threatens to move to c4 with strong effect, White plays b2-b3, protecting the square.
Black also has weak squares on a5 and h5. These are less important because they are further from the center, but they would require constant attention during a game if White started to invade the Black position via one of these squares.
Strong and weak squares are not determined only by the pawn structure. In the initial position, before either player has made the first move, White has a weak square at f2 and Black has a weak square at f7. These squares are weak because they are protected only by the King, who is not a fierce fighter until the later stages of the game.
