This is the 101st most frequent Vietnamese word.
gì
"What."
The word "gì" in this sentence means "what" and is used in a question to ask for specific information about an action.
Bạn đang làm gì?
What are you doing?
Here, "gì" is used after "cái" to form "cái gì", meaning "what", and it functions as a pronoun for something unspecified.
Anh ấy không biết cái gì đó đang diễn ra.
He doesn't know what is happening.
In this sentence, "gì" follows "cái" to form "cái gì", specifically asking for the identification of an object.