"Don't dream" means that we can't just see the superficial phenomenon of things, but should see the essence through the phenomenon. For example, when we see a leaf, we should not only look at its shape, color and texture, but also consider how it interacts with the whole plant system and how it performs photosynthesis and absorbs nutrients.
"Don't think too much" means that we can't just be confused by the beautiful appearance of things, but should think deeply about their intrinsic value and significance. For example, when we see a beautiful flower, we should not only appreciate its color and shape, but also think about how it interacts with the surrounding environment and ecosystem and what its contribution to ecology and biodiversity is.
In short, "not thinking about leaves, not thinking about flowers" is to warn us not to be confused by superficial phenomena, but to think deeply and explore the essence and connotation of things. Only in this way can we truly understand the value and significance of things and make more wise decisions.