Realism -- at least the extreme forms -- essentially denies the role of subjectivity in determining the final way something is perceived. As said above, color is a great example. There is no such thing as "red" out there, yet it isn't entirely "in here" either, but rather a synthesis of subjectivity and objectivity: I perceive a surface with light reflecting off of it, which interacts with the rods and cones in my eyes to determine what color something is. Relative rods and cones constitutions determines relative colors perceived by the individual. There ain't no such thing as color "out there" -- that's a ghostly world of Kantian things-in-themselves, which I believe does exist, but you can't really say anything about it, because it's a "qualityless existence," and we only know things according to their properties and qualities. "What we cannot speak about we must pass over in silence."
By extension, the rods/cones/color analogy can be applied to pretty much everything. Minimally, size determines how something is perceived and labeled. I see a phone; a molecule would only "see" other molecules, the concept of "phone" having absolutely no reality for this entity. This doesn't mean that there is no "real" out there, or that "real" is merely the interaction of subjectivity and objectivity (although the morphology of the term "exist" is interesting: "existere," "to show forth"), but that whatever *is* out there is "colored" by the constitution of the perceiver. Like Aquinas said, reality is presented in the mode of the perceiver.