John sees thrones but we are not told where they are located. Given that the description is similar to previous descriptions of the heavenly throne room, it is likely that John was shown what was taking place in heaven at that time.
He saw rulers, which may be a reference to angels, but more likely refers to believers who have died. Then he mentions those who had been martyred for the sake of Jesus (including John’s own brother James). They reign with Jesus during the thousand years.
Their coming to life is said to be the first resurrection, yet what is surprising about them is that John does not see their bodies. Instead he sees their souls. This also would suggest that the location of the thrones is heaven.
Here we have information about what the righteous dead are engaged in during this period of a thousand years. In heaven, they function as priests and kings. As priests, they participate in the worship of God and of Christ, and as kings they reign with Jesus.
We don’t know what or how those contributions take place. Yet we can deduce several details from the description. First, they are conscious, involved in the life of heaven. Second, they are consecrated to divine service. Third, they will experience the work of the Spirit – this is implied in their roles as kings and priests because such were anointed for their tasks. Fourth, they have communion with God and with Jesus.