Dissociation is a normal coping mechanism. You have experienced when you become overly focused on an activity you might not hear someone else calling you from another room. It becomes a disorder when you regularly feel like you are on the outside looking in, feeling disconnected, or in some cases watching yourself in daily life.
A dissociative disorder is generally associated with individuals who experience trauma. It is a way to keep intrusive memories of past events away from current life. It may include amnesia, memory loss, flashbacks, feeling disconnected, emotional numbness, an altered sense of reality.