An onomatopoeic/mimetic word describing things or people crowding, bustling, or wriggling together; often used adverbially as 'warawara to'.
公園に子供たちがわらわらと集まって遊んでいる。
Children are gathering and playing in the park in a bustling way.
夏になると海岸にカニがわらわらと出てくる。
In summer, crabs come out along the shore, crawling about in numbers.
イベント会場に人がわらわらと入ってきて席が埋まった。
People streamed into the event hall and the seats filled up.