신나다 is bubbling, visible excitement. The commonest line here is the teasing question 왜 이렇게 신났어?, which appears several times over — and note that 이렇게 means "this much," not "in this way." It is usually aimed at someone whose mood is suspiciously high: 이 형 쑥스러워하더니 뭐 이렇게 신나서 지금, 왜 이렇게 신나는 표정이죠, 불안하게?. One caption uses a different adverb: [족탁구 영재들은 그저 해맑게 신남], happily hyped.