Being zealous or enthusiastic; having keen interest or earnest attention toward something (na-adjective).
彼は勉強に熱心で、毎晩遅くまで図書館にいる。
He is serious about studying and stays in the library until late every night.
先生は生徒の質問に熱心に答えた。
The teacher answered the students' questions enthusiastically.
彼女は環境保護活動に熱心だ。
She is enthusiastic about environmental protection activities.