This is the 764th most frequent Serbian word.
они
"They" (third person plural pronoun).
In this sentence, 'они' is used as a subject pronoun in the nominative case for a group of people.
Они су дошли на време.
They arrived on time.
'Оне' (form of 'они') refers to an object, meaning 'those' in the sense of pointing to specific mountains, used in the accusative case.
Да ли си видео оне планине?
Did you see those mountains?
Here, 'они' is used as a relative pronoun establishing a clause to define a subject group.