What does John 9:21 mean?
After Jesus gives sight to man born blind (John 9:1–7), controversy erupts. This healing, as were many of Jesus' miracles, occurred on a Sabbath day (John 9:14). The local religious leaders, referred to as "the Jews" in the gospel of John, have made it clear that anyone who supports Jesus is subject to excommunication (John 9:22). As part of their effort to debunk Jesus' miracle, the scribes and Pharisees have summoned the parents of the formerly-blind man, asking if he was really their son, and really born blind. The parents have already testified to both of those facts.Here, the parents respond to some of the other questions raised by Jesus' miraculous act. The scribes and Pharisees repeatedly ask "how" Jesus has accomplished this feat. This shows their inherent prejudice: they cannot accept that Jesus is healing with divine power, so they seek some other explanation. The meaning of the miracle—the message behind the "sign"—is lost on them, since they refuse to believe in Jesus and His words (John 5:39–40).
The parents pointedly state that they don't know "who" healed their son. While this is almost certainly true, it might also be part of their effort to avoid trouble. As the next verse states, those who identify Jesus as the Messiah would be cast out of the synagogue. The healed man's parents are being extremely careful not to cross that line. The parents are also careful to point out that the man in question is a legal adult; he is capable of answering these questions for himself, so there is no reason to require testimony from his parents.