To put it in lay terms, dissociation is something that a lot of people do. For example, day dreaming or imagining you are someplace else (for example not at school, or work). People that experience trauma, or abuse can take this to an extreme and actually become someone else. Someone who is stronger, or create a partition for bad memories to stay so they don’t have to deal with them in the conscious mind. This used to be called MPD or Multiple Personality Disorder, but was changed years ago to be broader and is now a Spectrum diagnosis called Dissociative Identity Disorder or DID. It now covers mild but dysfunctional episodes to extreme changes which are sometimes non-functional for daily social interaction. For more clinical terms see the links below.