This is the 445th most frequent Russian word.
какой
"Какой" means "which" or "what kind of."
Here, 'какой' is used as an interrogative pronoun to inquire about a specific attribute or choice.
Какой цвет тебе нравится?
What color do you like?
In this case, 'какой' serves to introduce a subordinate clause and expresses uncertainty regarding the choice.
Я не знал, какой фильм выбрать.
I didn't know which movie to choose.
This usage of 'какой' expresses an exclamatory statement to emphasize a quality of the subject.