Everyone has experienced some psychologically traumatic event in their life (from something mild like being embarrassed in front of colleagues to severe such as being sexually assaulted). Your brain also has a mechanism that “heals” the impact of a traumatic event.
This healing is a process your mind goes through so that the visceral feelings attached to a traumatic event get removed. Then, when a memory gets stored in your long-term memory, it is simply a memory but is not one that carries with it a strong sensitivity, reactivity, nor influences much of your current behavior.
However, your brain is an imperfect system, and some memories slip through the “healing” process. Some memories are just so intense that not all of what needs to be healed gets healed before that memory gets sent to long-term memory storage.
Thus, a memory of an event becomes unresolved. This means that when recalling that memory also lead the person to feel some of the same feelings they felt during the traumatic events.
For example, if someone had an unresolved memory of watching a parent die, when recalling that memory, they still may feel intense fear, helplessness, and emotional pain.
This differs from a resolved trauma. For example, let’s say that same person was also in a severe car accident in their teens. Yet, when they recall that memory they do not have the same feelings of fear or panic (though it certainly is not a pleasant memory).