I don't understand the meaning of 'ci' when a person say: "Che ci fai qui?". It seems redundant the use of 'ci' and then the use of 'qui'.
1In a comment to this question I am listing other questions about ci. I am not saying this is a duplicate. Would you like to have a look at those questions and see whether any of them answers your question?– DaGOct 9, 2019 at 12:43
3On a related but distinct note: sooner or later, someone should write (or transcribe from some text) the ultimate answer about ci, and make it the Über-FAQ.– DaGOct 9, 2019 at 12:44
@DaG great idea.– Easymode44Oct 10, 2019 at 12:25
In this sentence, "ci" has the meaning of "in questo luogo" or "qui". It is used in a pleonastic way because the sentence in your question contains a "dislocazione a destra", a construction typical of oral speech which is explained in detail in this answer.
Your sentence has indeed a similar structure to this example
Non ci sono andato, a Venezia
given in the book Grammatica dell'italiano adulto by Vittorio Coletti: there is a place complement ("a Venezia" in Coletti's example, "qui" in your sentence) that goes after a complete clause and which is anticipated in this clause by particle "ci". Another example with the same phenomenon is explained in the answers to this question.