This is the 386th most frequent Polish word.
kto
"Who"
'Kto' is used as an interrogative pronoun to ask about a person.
Kto to jest?
Who is this?
'Kto' introduces an indirect question about the subject of the action.
Zapytałem, kto przyjdzie na spotkanie.
I asked who will attend the meeting.
'Kto' is a relative pronoun in this case, specifying the person.