"Collective unconscious is a term of analytical psychology, coined by Carl Jung. It is a part of the unconscious mind, expressed inhumanity and all life forms with nervous systems, and describes how the structure of the psyche autonomously organizes experience...the Collective Unconscious collects and organizes those personal experiences in a similar way with each member of a particular species."
Basically, the collective unconscious is a part of the brain which holds past experiences as far back as when you were in the womb. These experiences surface mainly during dreams. I find this to be so fascinating that our minds can retain information that we couldn't remember if we tried. If someone asked me what it was like in the womb, I could never be able to describe that to them, but it doesn't mean i don't have the memories stored away...fascinating!