This is the 8378th most frequent Japanese word.
快活
"快活" means "cheerful" or "lively."
Here '快活' is used as an adjective to describe someone's personality, denoting cheerful and lively behavior.
彼女は快活な性格で、いつも明るい笑顔を見せます。
She has a cheerful personality and always shows a bright smile.
In this instance, '快活' is used as an adverbial noun to imply the manner of behaving cheerfully.
快活に振る舞うことで、周囲の雰囲気を和やかにすることができます。
By acting lively and cheerfully, you can create a harmonious atmosphere around you.
Here, '快活さ' is the noun form of '快活', modifying the individual's ability to uplift others.