The scales were blinding the person, and when they fall from the eyes, the person can see clearly.
It's a reference to the Bible story of Saul becoming Paul. God strikes Saul blind by putting scales (as in karatin, not a weight-measuring device) on his eyes. Later, when Saul decides to stop persecuting Christians, the scales start to fall from his eyes, he regains his sight, and changes his name to Paul.