What does it mean that the Jerusalem which is above is free and not the earthly Jerusalem?
The Jerusalem above is the spiritual, heavenly reality, not the earthly city where the temple still offered sacrifices. It is free because believers are delivered from darkness and translated into the Son's heavenly kingdom, made partakers of the saints' inheritance in light, with their head and citizenship in heaven.
Les explains that the Jerusalem which is above is contrasted with the earthly Jerusalem where the temple was still carrying out her sacrifices. The believer's setting is now spiritual in the heavenlies, and it is free. He connects this to Colossians 1:12-13, where Paul writes giving thanks to the Father, who made us meet to be partakers of the inheritance of the saints in light, delivered us from the power of darkness, and translated us into the kingdom of His dear Son. That kingdom is in heaven, so the believer now has a heavenly base of operations. The Head of the body is in heaven, and we are citizens. Les then turns to Philippians chapter 3 to tie it together.
But Jerusalem which is above is free, which is the mother of us all.