Over on The Motley Fool atheist board, someone recently posted the question: "Could somebody please explain the term 'objective reality' to me?"
We don't view the world as it really is; we interpret it with our senses and filter it through existing patterns in our brains. For instance, when I think that I see a blue racquetball, I am not really perceiving the ball directly. White light is striking the surface of that ball, all the wavelengths are absorbed except for those that we recognize at the color blue, and then the light bounces back to the surface of our eyes. The cells in our eyes transmit the pattern of photons back to our brain, which then looks at the pattern of light and dark shading, interprets the slightly different information from each eye to estimate distance, and then creates sort of a computer simulated model of a ball. Your brain tells you "That's a blue ball!" and that's what you think you see.
But senses can be fooled or misled, and your brain's program can screw up and misinterpret what it's reading. Then you can get a false impression of what you are seeing in the world.
Furthermore, you interpret a lot of things based on your memories of things that have happened to you in the past. If you see or hear about something that conflicts with the world model that was already in your head, you might reject the new information or file it wrong in your memory, because your brain doesn't like to completely reorganize its existing patterns every time it sees something a bit odd.
So there's a "real world" out there, outside your brain; and then there's the "virtual world" that has been built inside your brain. The real and the virtual world never match up completely, but they can correspond to a greater or lesser degree. When you see a blue ball, you can be pretty confident that there really is a ball and it really has the property of being blue. The color blue is not really a "thing"; it is just a word that we use to label light at a certain wavelength. But there really is light, and it really has different wavelengths, and it really does bounce off of things like balls to show you the color blue.
When we talk about "objective reality", we are talking about the world that's really there, unfiltered, outside your mind. Our beliefs do not change the world, except to the extent that they lead to actions that alter reality. So I can, if I try hard enough, go around all day sincerely believing things like "That blue ball is actually an orange artichoke" or "There's an invisible man living in the sky who watches everything you do, every minute of every day" or "The British government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa." But if those things are not correct statements about the real world, then no amount of belief will change that.