This is the 1791st most frequent Russian word.
каких
A form of "какой," meaning "which" or "what kind of" in plural and genitive/accusative cases.
The word 'каких' is a declined form of 'какой' that corresponds to 'which' or 'what kind of' in English, and it's used here as an interrogative word referring to plural nouns in the genitive case.
Каких книг вы читали в детстве?
What books did you read in childhood?
The word 'каких' functions here as a relative pronoun introducing a subordinate clause, specifying the kind of people and appearing in the genitive case to match the noun.
Я не знаю, каких людей ты встречал.
I don't know what kind of people you have met.
In this sentence, 'каких' acts as a relative pronoun referring back to a plural antecedent in the genitive case, emphasizing the notable characteristics of the persons remembered.