This is the 1st most frequent Indonesian word.
yang
"Yang" means "which," "that," or "who" and is used to introduce relative clauses.
'Yang' is used to introduce a relative clause, specifying what kind of friend is being referred to.
Dia adalah teman yang selalu membantu saya.
He/She is a friend who always helps me.
'Yang' is used to emphasize or clarify the subject of the verb in the sentence.
Saya tidak tahu siapa yang mengetuk pintu tadi.
I don't know who knocked on the door earlier.
'Yang' serves to identify or specify the particular book being discussed.