さ is a suffix in Japanese that can be attached to the stem form of
い-Adjectives and
な-Adjectives. When this happens, it transforms the target adjective into a noun that illustrates the ‘amount’ of that particular adjective. This commonly translates as ‘-ness’ in English (sweetness, tallness, spiciness, etc), but is called 程度（ていど） ‘amount’ in Japanese.
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私（わたし）が道（みち）の長（なが）さを測（はか）ります。
I will measure the length of the road. (The road’s lengthiness)
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大人（おとな）になってから家族（かぞく）の大切（たいせつ）さが分（わ）かってきた。
After becoming an adult, I came to understand the importance of family. (Family’s importantness)
It is thought that this suffix originally stemmed from the kanji 差（さ） (meaning ‘distinction’), and was used for highlighting things that were distinctive features of something. In this way,
さ is also sometimes used after the dictionary form of verbs, to highlight a distinct point in time that something happens.
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学校（がっこう）からの帰（かえ）るさにお金（かね）を拾（ひろ）った。
On the way back from school, I picked up some money. (At the ‘point’ of returning home)
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行（い）くさで家（いえ）に忘（わす）れ物（もの）をしたことを思（おも）い出（だ）した。
When I was on my way there, I remembered that I had left something at home. (At the ‘point’ of going somewhere)
However, this use of the suffix is far less common in modern Japanese, and is known as 雅語（がご） ‘poetic language’. It was primarily used during Japan’s ‘middle ages’, which spanned from (around) 1100, until close to the end of the 1500’s.