This is the 905th most frequent Korean word.
아이들이
"아이들이" means "children" (subject form).
Here, '아이들이' is the subject of the sentence, meaning 'children' in a specific or general sense, as indicated by '이', the subject particle.
아이들이 공원에서 놀고 있습니다.
The children are playing in the park.
'아이들이' serves as the subject of the sentence, focusing on the action performed by the children.
아이들이 학교에 갑니다.
The children go to school.
In this sentence, '아이들이' indicates the possessive or topic marker about what children (아이들이) like, tied with the relative clause.