This is the 2297th most frequent Korean word.
활짝
"Completely open" or "widely."
Here, '활짝' is used to describe the state of the flowers opening completely.
꽃이 활짝 피었다.
The flowers have bloomed fully.
In this sentence, '활짝' characterizes the broad and cheerful nature of her smile.
그녀가 얼굴을 활짝 웃었다.
She smiled brightly.
'활짝' in this case describes the fully open state of the door.