This chapter is a pause in the judgment. Seven angels with seven plagues are introduced. They will complete the judgment.
Those who overcome the beast and his image and his number stood on the sea. Overcoming the beast I understand, overcoming his number and his name must speak to the completeness of their rejection of and victory over the beast. You overcome the image by not worshiping it and overcome the number by refusing it.
They sung the song of Moses and the lamb. It is not the song of Exodus 15, though both songs reflect God's great victory!