Symbolic thinking describes the process of creating an internal representation of a situation, scene or thought. We use all of our senses to imagine a situation and create our own internal representation. Every person is different so that everyone creates his own emotional reaction about the scene.
Understanding the concepts of symbolic thinking makes you a better graphic recorder as you can use those metaphors to tell more engaging stories even about the most boring topics. Just ask what the topic means for the people in your story and find the right metaphor.