This is the 386th most frequent Polish word.
kto
"Who"
'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.
On był tym, kto pomógł mi tego dnia.
He was the one who helped me that day.
'Kto' is used as an interrogative pronoun to ask about a person.