Refers to the quality or state of being weak, whether physical, mental, or abstract. Often used in contrast with 強さ (strength).
彼の弱さは優しさでもある。
His weakness is also his kindness.
この素材の弱さが問題だ。
The weakness of this material is the problem.
弱さ means weakness or the degree of being weak, the direct opposite of 強さ, which means strength.
Derived from the adjective 弱い (よわい, 'weak') by adding the nominalizing suffix さ, which turns an adjective into a noun expressing the degree or quality of that adjective.