A fear hierarchy is a catalog or list of situations or items that bring on fear or anxiety. For someone with PTSD, these would be situations or items associated with a traumatic event. The items on the list are ranked from being the least anxiety provoking to the most anxiety provoking.
A fear hierarchy is used in exposure therapy for PTSD to gradually decrease the extent with which someone avoids trauma-related triggers. For example, a rape victim may start off by watching a television program that depicts a sexual assault, gradually moving up the fear hierarchy to actually visiting the place where they were assaulted.