Who Are the 'Righteous Ones' in the Parable of the Lost Sheep?

A listener named Barbara asks Father Dave about the Parable of the Lost Sheep. She says, "In the Gospel Parable of the [Lost Sheep], Jesus says there will be more joy in heaven over one sinner who repents than over 99 righteous people who have no need of repentance. Who is he referring to when he says righteous people, and what makes them righteous?"