Here's an example of dissociation: A person who was just involved in a serious car accident (an example of trauma) may find that in the days following the accident, they are "spacing out" more than usual, getting lost in their own thoughts and not realizing that hours have gone by.
They may have distressing thoughts of the accident instead of being able to focus on their daily tasks. Or, they may feel numb and unable to experience emotion after the trauma. In this case, the person may find that their dissociation fades after a few days, or they may notice that their dissociation reoccurs.
When dissociation after trauma is reoccurring, it may be a sign that someone is experiencing PTSD.