Perestroika continues the angel motif that began in Part One: Millennium Approaches (read about it here). There the angel motif is mostly metaphorical, but at the end of Millennium Approaches, the motif becomes pretty darn literal when an actual angel comes crashing through Prior Walter's ceiling.
The angel puts in several more appearances in Perestroika, and we learn more about her. We find out that she represents the Continental Principality of America. When Prior goes to Heaven we meet the angels that represent the other parts of the world – Antarctica, Australia, Europe, etc.
We also find that she and the other angels are actually not shes or hes at all. They're all hermaphrodites equipped with both male and female sex organs. We also learn that the angels had sex constantly at the dawn of time; their orgasms evidently fueled the engine of creation. (We know, it's a little weird.) Among other things, this detail connects the motif of angels to the theme of sexuality. Because the play focuses on AIDS, a deadly sexually transmitted disease, sex is linked to death quite a lot. The angels' cosmic copulation, however, reminds us that sex is the ultimate act of creation.
It's also important to note the job the angels want Prior to do – to make humanity stop growing, changing, and progressing. Prior ultimately rejects this task and tells the angels that it's just not possible for humanity to stop in its tracks. It simply goes against our nature. Kushner portrays the angels as radically conservative – conservative on a cosmic level – because they are against change and progress. When Prior rejects their charge, he rejects their brand of cosmic conservatism.