The ultimate shared dream is a waking experience. Culture is, essentially, a series of stories that we all experience together and make our lives by sharing with one another. Some people take some stories as true or important, while others do not. The key thing is, these stories are in our minds, yet, like dreams, we take them to be real.
For example, some people think the lives of celebrities are important/interesting. So, they watch shows about them, read magazines, and so on. This is a cultural phenomenon (one of MANY), where they read or watch a story (or acquire it through conversation, etc.) that others are also reading or watching. They react to these stories emotionally, their thoughts are involved in them. The thing is, whatever Britney is doing is something they never actually experience, except as a mental image, just like a dream.
In fact, much of our lives are lived in our minds, probably most of our lives, if you counted the seconds we devote to thinking and fantasizing, as opposed to other activities. The content of Culture and Society, however, while never experienced as anything but thoughts (thoughts about real experiences, sometimes, but never experienced in themselves) are taken to be real things, actual experiences. This is a very basic illusion about reality that most of us don’t see, and take for granted as real.
The Rev