What does John 9:20 mean?

These parents have been called before a council of Jerusalem's religious leaders. In the gospel of John, this cadre of scribes and Pharisees, and those who follow them, are referred to as "the Jews." The reason for their summons is a miracle Jesus has performed: giving sight to a man who had been blind from birth (John 9:1–7). In an effort to investigate—and hopefully to discredit Jesus—the religious leaders interrogate the man's parents to see if the person claiming to have been healed really is their son, and if he really was blind his entire life.

The parents' response is timid and short for a reason. Verse 22 will explain that the religious leaders of Jerusalem have already declared a punishment for anyone who follows Jesus: excommunication. The formerly-blind man's parents seek to tell the truth, but they're clearly not willing to say anything beyond the bare facts which they, themselves, can know. Their testimony will continue in the next verse, including their plea for the interrogation to focus on their son.