When Andrew brings Simon to Jesus, Jesus announces that he will be called Cephas. John immediately interprets the name for Greek-speaking readers as Peter.
The verse preserves the bilingual environment of the early Jesus movement: an Aramaic name is carried into a Greek Gospel and translated.
Cephas represents Aramaic kepha, meaning rock or stone. The Greek Petros likewise functions as the corresponding name Peter. Paul also uses Cephas repeatedly in his letters, showing that the Aramaic form remained an important early designation for Simon.
John does not explain the entire significance of the naming in this verse, so later Petrine roles should not all be read back into the brief scene as though explicitly stated here.
Recognizing the translation makes clear that Cephas and Peter are not two unrelated names or two different people. They are Semitic and Greek forms of the name given to Simon.
The act of naming also signals that Jesus claims an unusual authority to define Simon's future identity before Simon has yet done anything in John's narrative to earn the designation.