Moon Cliff is a massive cliff located on the border of The Pack of Falling Stars and The Pack of Night's territory. It's shown to have a long rigid trail that coils around most of the peak, leading up to a dusky flat surface where wolves often howl.
In this episode Fang is shown howling, and talking to Flutter after she dies; this is most likely because in earlier episodes Flutter took Fang there and asked if he could hear the voices, saying that this was where deceased wolves would speak to the living ones.
In Episode 21, Luna explains that theres no prey, no one in the pack knows why Fang keeps it. However, veiwers can guess that Fang keeps it as part of their territory because he doesn't want to let go of Flutter.