Realism is the view in metaphysics that the physical world truly exists, and is separate from or our perception of it. With respect to epistemology, realism is the view that we directly perceive the world as it is, or things in themselves, through our senses. The world inside our minds is identical to the world as it is — what we see, feel, taste, and so on, is accurately how the world is.
The directly oppositional position to realism in the metaphysical sense is idealism, which holds that the mind is the only substance to exist. However, there are epistemological positions that do not deny the existence of reality, but are opposed to epistemic realism (that we perceive the actual world). The most prominent of such positions is the transcendental idealism of Immanuel Kant.