The angel had been talking to John since Revelation 17:7. In Revelation 17:8–14 the angel revealed the mystery concerning the beast, including its seven heads and ten horns. In Revelation 17:15–18 the angel shifts focus to reveal the mystery concerning the woman. The opening words of Revelation 17:15 are there to indicate that shift.
15 And the angel said to me, “The waters that you saw, where the prostitute is seated, are peoples and multitudes and nations and languages.