The Hierarchy of Fears is a detailed list of stimuli that trigger a fear response in people. It is designed for use in desensitizing people to the things that trigger their fears, ultimately helping them overcome their fear in little steps.
The list is organized from the things that produce the least intense fear response (at the top of the list) to the things that produce the most intense fear response (at the bottom).
For example, let’s say a solider suffering from PTSD (Post Traumatic Stress Disorder) is now terrified by guns. Even a picture of a gun can elicit a fear response in the solider. Handling a gun could cause a serious fear response. In this case, the solider could choose looking at pictures of guns as the least intense fear (top of the list) and holding or shooting a gun could be the most intense fear (bottom of the list).
Related: Systematic Desensitization