This is the 4193rd most frequent Russian word.
тко
The word "тко" does not have a meaning in Russian; it may be a typo or misspelling.
In this sentence, "кто" is used as the interrogative pronoun "who" to ask a question about the subject of the sentence.
Кто идет в парк?
Who is going to the park?
Here, "кто" functions as a relative pronoun introducing a subordinate clause elaborating on the preceding part of the sentence.
Я даже не помню, кто мне это сказал.
I even don't remember who told me this.
In this case, "кто-то" is an indefinite pronoun meaning "someone," expanding the usage of the base form "кто."