This is the 432nd most frequent Indonesian word.
apa
"What."
In this context, "Apa" functions as a question word used for introducing a general inquiry or greeting, equivalent to "How" in English.
Apa kabar?
How are you?
Here, "Apa" is used to introduce a subordinate clause, equivalent to "what" in English, referring to an object or an idea in question.
Saya tidak tahu apa yang terjadi.
I don't know what happened.
In this usage, "Apakah" (derived from "Apa") is a formal interrogative particle used to form yes-no questions.