The unconscious or subconscious mind is a part of the mind which stores repressed memories. The theory of repression maintains that some experiences are too painful to be remembered, so the mind suppresses them in the "cellar". These painful, repressed memories manifest themselves in neurotic or psychotic behavior and in dreams. At least that's what some people think.
Others view the unconscious or subconscious as the home of archetypal patterns, dream states, the imagination, spirit beings and all the aspects of our consciousness not normally considered part of consensus reality.