This さ turns adjectives into nouns, expressing the abstract quality or degree of that adjective. It's like adding '-ness' in English. For い-adjectives, you drop the final い and add さ. For な-adjectives, you add さ to the stem. This is a very common and productive pattern, and the resulting nouns are used in everyday conversation and writing.

Be careful: this is not the same as the sentence-final particle さ, which adds a casual, assertive tone (like 'you know'). That さ is a separate grammar point. Also, さ is different from み (as in 楽しみ, 悲しみ), which creates more concrete or personal feelings. さ is about measurable or observable degree.