The metaconscious mind was an aspect of the Betazed psyche. It acted as a filtering mechanism for thoughts and memories that remained otherwise highly traumatic and debilitating.
In 2370, Ambassador Lwaxana Troi was reminded of her deceased daughter Kestra, during a diplomatic and indoctrination mission with a race of telepaths called the Cairn. The remembrance triggered the release of painful memories that Lwaxana had suppressed for years within her metaconscious mind, and it was only through the empathic and telepathy-assisted explorations of her daughter Deanna that she was able to reconcile them. (TNG: "Dark Page")