What is the Jerusalem that is above in Galatians 4?

The Jerusalem that is above is the heavenly situation, the free Jerusalem, of which believers are now citizens. It represents grace, pictured by Isaac, in contrast to the earthly Jerusalem that is in bondage to the Law. Scripture calls it free.

Les explains that the Jerusalem which is above is the heavenly situation of which believers are now citizens. He notes that Paul writes that we are citizens of the heavenlies. This free Jerusalem corresponds to grace, pictured by Isaac, while the earthly Jerusalem that now is, in bondage with her children, corresponds to the Mosaic Law and Hagar. The allegory contrasts the two covenants: Mount Sinai, which genders to bondage, and the free woman, representing promise.

Galatians 4:26

But Jerusalem which is above is free, which is the mother of us all.

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.