What does "cast the first stone" mean?

Also, throw the first stone. Be quick to blame, criticize, or punish, as in She's always criticizing her colleagues, casting the first stone no matter what the circumstances. The term comes from the New Testament (John 8:7), where Jesus defends an adulteress against those who would stone her, saying “He that is without sin among you, let him first cast a stone at her.” Also see people who live in glass houses; pot calling the kettle black.