Essentially, it means that if something is real, it must be visible, hence, physical. All reality can be reduced to, or is explainable in terms of, physical matter. As a conceptual principle, it cannot be visible to the eye or detectable by any of the rest of the senses. In fact, our existence depends upon them. Moral values such as justice and goodness exist, but they cannot be seen.
Truth is an invisible reality.
So is love. Feelings and emotions surely exist, but they cannot be observed directly. A central weakness of all forms of materialism, including the simplistic form adopted by many young students, is its inability to account for the transcendent components of life, including the cognitive or conceptual elements. We cannot see our thoughts. And though we can see the human brain, the mind is unobservable.