The phrase communicates the thought that what John sees next happens as a consequence of the things John had just seen and recorded. In other words, the Lord God responds to the song of the conquerors in his presence with the activity John now sees occurring. These verses provide John (and his readers) with detail about how God will use the plagues to compel “all nations” to “worship” him.
5 After this I looked, and the sanctuary of the tent of witness in heaven was opened,