Refers to the quality of being light in weight, or figuratively light in feeling or manner. Often contrasted with 重さ (heaviness).
See also: 重さ
I liked the lightness of this material.
There is an astonishing lightness in her movements.
軽さ means 'lightness', the direct opposite of 重さ ('weight/heaviness').
Derived from the adjective 軽い (かるい, 'light') by adding the nominalizing suffix さ (-sa), which forms abstract nouns expressing degree or quality.