This is the 13th most frequent Polish word.
co
"What"
Used as an interrogative pronoun to ask a question about an activity.
Co robisz teraz?
What are you doing now?
Used as a relative pronoun to introduce a subordinate clause indicating the thing in question.
Nie wiem, co mam zrobić.
I don't know what to do.
Used as a relative pronoun, specifically connecting a qualifier to the antecedent 'everything'.