Not to be confused with optimism or happy thoughts--which I also believe in--philosophical idealism is the idea that all our perceptions are mental phenomenon, rather than physical, material, external, phenomena. They are "in the mind" just like dreams. But they are real. Idealism defines reality in a very simple way: reality is what you feel, hear, see, smell, and experience. That's it. No abstractions. A tree is how we perceive it.
Sounds crazy but think about it. While you are dreaming you have perceptions that are real. You see, feel, walk, hear, and have a range of emotions. They are, of course, mental, in your mind, yet real. So why can't reality be like that?
According to George Berkeley (and me) reality is exactly like that. And God is the dream maker. We live in God's dream, God's Matrix. When we wake, we exit our world and enter Gods. It's uniform, coherent, mathematical, beautiful. When we look at a tree, we are looking directly at one of God's ideas; or, God is implanting an experience in us. When we do science, we are uncovering the laws and rules of the Mind of God, the Grammatical Rules of the Author of Nature. Malebranche said we "see all things in God" and he meant it. For those who believe in God, it's an incredible, magnificent idea. Jesus said that people will look for the Kingdom of God, asking "Is it up there? Is it over here?" No, the Kingdom of God is within you, but men do not see it. God is present and immanent in a way we didn't even expect, "in whom we live, move, and have our being."
So does a tree make a noise if nobody is there to hear it fall? Yes, but only because God is there to "hear" it.
The metaphor is tempting. Also, you cannot really disprove it. That's the real bitch of it. We really could be living in the Matrix right now. We also could be living in God's dream. Or neither. Just admitting that it's a possibility is amazing from an intellectual standpoint. So much for certainty! The point is not to denigrate the amazing world we live in; it's to help make sense of why it's so amazing, to put it in a larger context. As Emerson said, Idealism looks around and feels that the universe is, somehow, at bottom, in its essence, mental, spiritual, mind-like. Matter is an expression of mind, not the other way around.