This is the 10454th most frequent Russian word.
весело
Fun, cheerful, or merry.
In this sentence, 'весело' is used as an adverb, describing the atmosphere during the holiday celebration as cheerful and enjoyable.
На празднике было очень весело.
The holiday celebration was very fun.
Here, 'весело' modifies the verb 'играют,' illustrating the joyful manner of the children's activity.
Дети весело играют в парке.
The children are playing joyfully in the park.
In this example, 'весёлыми' is the adjective form of 'весело,' describing the stories as amusing and entertaining.