This is the 435th most frequent Korean word.
이들
"이들" means "these" or "these people."
Here, '이들' refers to a group of specific individuals being introduced as friends.
이들은 나의 친구들이다.
These are my friends.
In this sentence, '이들' denotes a group of people whose actions are being described related to their arrival.
이들은 어제 도착했다.
They arrived yesterday.
'이들' is used here to refer to the individuals being as subjects participating in the action of studying.