Hi Soheil, I think this is more a chess question than an English question. In chess, weaker pieces are those that have more limited movement, for example, a rook is weaker than a queen because it can only move to the left or right, while the queen can also move diagonally. The text is saying that although this is the case, there are many instances where weaker pieces may be used to advantage against stronger ones.
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He could have used some other words. Why did he use 'weaker'? After all, force is given by pieces. When a pieces is weaker, it has less force.
W: Kb5, Pd7
B: Rc8, Ka7
White is to move