What does Christof mean when he says " We accept the reality of the world with which we're presented. It's as simple as that".

Simply, we accept the reality of the world at its face value. Reality is what we accept reality is. Truman, for much of the film, does not question his reality as artificial. Similarly many of us accept the media construct of our own reality. We buy things to make our reality meaningful. We strive to look like the narrow interpretation of beauty we have been fed by others. 

This question is simply answered through direct realism theory. Basically, it’s the idea that we accept what our senses are delivered. Christof’s shaped the environment so Truman is constantly fed sensory normality, but in actual fact, he was surrounded by TV sets and actors.