Carl Jung coined the term "collective unconscious" and this and his other concepts are increasingly becoming en vogue, again.
I think it's not surprising that the concept has become more relevant in the age of social media. The unconscious mind is anything that is not currently conscious. That is, those experiences, memories, or objects that have been forgotten or repressed. The idea is that our unconscious mind dictates many of our emotions, reactions, and overreactions to present day situations. Much of the unconscious is determined by our unique life experiences. For example, if we were abandoned by our parents at an early age, we might consistently fear the pain of abandonment in the future and guard against this by either being extremely doting in relationships, or, alternatively rejecting relationships before we ourselves can be rejected.
However, the (potentially) more powerful part of the unconscious is the collective unconscious. It is one that is shared by all people in a society. This is the inherited part, the one that comprises the unconscious lives of our ancestors, right back to the earliest beings. It creates a latticework of all ancestral life experiences, and we are often not aware of it. But it exerts a strong power over us. Think about it, how the stories passed down from our ancestors shape us. Think about the unconscious messages we share all of the time. It is an incredible force, one that can cause incredible destruction (think Nazi Germany) and incredible growth (think manifest destiny). Of course, sometimes the very thing that leads to growth later leads to destruction.
I think that the collective unconscious is what is tapped into when a book resonates with a large number of people. I imagine Dostoyevsky as being a master of this. What he writes just 'feels true' and I don't mean factually true. The unconscious is deeper than fact, it resonates, it feels instinctual.
Think about the hero's journey. There is a call to action, hero resists the call, then he must heed the call, he is challenged, loses almost everything, then triumphs. If a story does not follow this narrative, something feels off, it feels wrong, we feel cheated. Yet, most of reality does not follow this narrative. In fact this narrative, on average, is not factual!! But if the narrative is not followed, it feels false. This is the unconscious mind's barometer.
So next time you're thinking about a novel or movie long after you shut the book or leave your seat, think about your unconscious mind, what that story tapped into, and which ancestor's story it is highlighting...