Refers to physical warmth (temperature) or emotional warmth (kindness, friendliness).
春の日差しの暖かさが気持ちいい。
The warmth of the spring sunshine feels good.
彼女の言葉には暖かさがある。
There is warmth in her words.
Noun form of the adjective 暖かい (warm), derived by adding the suffix 〜さ (-sa) to the stem, which turns an adjective into a noun expressing degree or quality.