It is the remedy for any fear whose cause can't be named. At one end of the spectrum, the Aspen fear may be no more than a sense of foreboding, an uneasy anxiety that something unpleasant or frightening is going to happen. At the other, it can be a real terror, with physical symptoms such as the hair standing on end.
Sometimes people think of Aspen as a night-time fear, related to dreams and night terrors. Aspen would certainly be the right remedy if you were lying awake in the dark scared of something but not knowing what it was. But in fact, Dr. Bach said that fear of the dark is a Mimulus fear since the cause (the dark) can be named. And nameless Aspen fears are just as likely to occur in full sunlight as on a dark night.