This is the 1967th most frequent Korean word.
마음껏
"마음껏" means "as much as one wants" or "to one's heart's content."
마음껏 implies doing something without holding back or freely.
마음껏 먹고 싶은 음식을 드세요.
Enjoy eating whatever food you desire.
Here, 마음껏 is used to express unrestricted action in playing.
아이들이 놀이터에서 마음껏 뛰어놀았다.
The children freely ran and played in the playground.
마음껏 in this sentence emphasizes doing something to one's fullest extent or potential.