This is the 8040th most frequent Japanese word.
嬉々
"嬉々" means "joyful" or "gleeful."
Here, '嬉々として' indicates that the subject is excited and happy about the action they are undertaking.
彼は嬉々として新しいチームに参加しました。
He joined the new team with excitement and joy.
In this sentence, '嬉々とした' describes a smile that is full of joy and happiness.
嬉々とした笑顔で彼女は部屋に入った。
With a joyful smile, she entered the room.
Here, '嬉々と' is used to describe the cheerful state in which the action (playing) is being performed.