This is the 178th most frequent Slovak word.
kto
"Who"
The word 'kto' is used here as a question word to inquire about the identity of a person.
Kto príde na večierok?
Who is coming to the party?
In this sentence, 'kto' functions as a relative pronoun introducing a subordinate clause.
Neviem, kto to bol.
I don't know who it was.
Here, 'kto' is used as an interrogative pronoun asking about a subject (someone's readiness).