TL;DR. Symbols are a part of life. Every human action is potentially symbolic. The clothes we wear, the food we eat, the way we talk may all reveal something about what kind of people we are. The same rule applies to fiction: the symbols present in a story might reveal something about the personality of the characters or the overall theme of the tale. This is natural symbolism.

Some writers are not satisfied with this kind of symbolism though. They want their stories to have a "deeper meaning". So they impose a framework on their story, hoping that with it comes the circumspection of High Art. Some can pull it off, but most can't. This approach also creates two problems: 1) People stop producing new symbols and 2) It results in stories being too samey.

So the final advice the author gives is to listen to life. Pay attention. You don't need to understand everything before sitting down to write. Your story will mean something to someone. It's better if that meaning comes from yourself looking at life than from yourself trying too hard.

All of this does not apply if you're James Joyce.