This is the 4815th most frequent Russian word.
играя
"Playing."
Here, 'играя' means 'playing' and is used as a gerund to describe an action simultaneous to forgetting.
Играя на пианино, он забывает обо всём.
Playing the piano, he forgets about everything.
In this sentence, 'играя' translates to 'while playing' and describes the activity of the children parallel to laughing.
Дети смеялись, играя в парке.
The children were laughing while playing in the park.
'Играя' here means 'while playing', suggesting an activity occurring during the state of happiness.