This is the 64th most frequent Russian word.
кто
"Кто" means "who" in English.
In this sentence, 'кто' is used as an interrogative pronoun to ask about the identity of the subject.
Кто в комнате?
Who is in the room?
In this use, 'кто' acts as a universal pronoun to express a general condition or rule.
Кто не работает, тот не ест.
He who does not work, shall not eat.
'Кто' here functions as a relative pronoun introducing a subordinate clause referring to an unknown subject.