This is the 377th most frequent Italian word.
chi
"Chi" means "who" or "whom" in English.
Here, 'chi' functions as an interrogative pronoun, meaning 'who', asking about the subject of the action.
Chi ha parlato durante la lezione?
Who spoke during the lesson?
In this sentence, 'chi' is also an interrogative pronoun, used within an indirect question to refer to the person responsible.
Non so chi abbia preso il mio quaderno.
I don't know who took my notebook.
Here, 'chi' is a relative pronoun that means 'whoever' or 'the one who', describing people in general who fulfill a condition.