The second half of Carl Jung's little collection of essays (only two in this small tome compared to his library of total work) is what the title says it is: Jung's explanation of why we dream and what it is that we see when we do.
There are some incredibly fascinating ideas in this essay, the jist of it is that we dream to give our minds a sense of what is occurring in reality, and this happens with various symbols that humanity has used throughout it's history. Of course, there are the obvious questions: how did we learn these symbols if no one taught us? If dreams are telling our minds something, then our minds are not source of the dreams, what exactly is?
"Where do dreams come from?" is an interesting question since there really is no answer outside of guessing, and every guess is equal to the other. Saying dreams are the result of random firings of our neurons as a sort of collection of knowledge from the last time we were asleep is equally as silly as saying that dreams come from somewhere outside us. There is really no way to know, and for all we know perhaps the truth is a mix of both reasons somehow we can't really articulate at this stage of our species development.
To shift focus to a slightly less mysterious point, the concept of symbols in our lives is a fascinating topic for the main reason that they're a paradox: symbolism is important to our psyche, however their interpretation is simultaneously objective and subjective. The symbol of a flood objectively means a chaotic cleansing, however whether that's a good or a bad cleansing is up to the individual.
I could keep writing on what I think Jung meant, however since no one can exactly ask him to elaborate on this matter, I want to give my own thoughts on the concept of symbolism: it's proof that art and aesthetics are vital to the human experience as well as the simple act of not being miserable.
There are three absolutes in every human's life: birth, an experience of lights and colors and sounds, and then death. You really don't have much say in the first and last of those, but you do have a say in how you interpret that middle part. Are those lights and sounds beautiful? Do they make you feel good? Honestly, that's up to you. We are all looking at the same painting in a grand museum, whether or not it's pleasing to you is really depending how you view it.