Idealism holds that the world is real, but it's subjectively real. Its reality is like that of pain. Pain is real, but it's completely in the mind of the one who's experiencing it. It's reality is subjective. If there were no creatures on earth whose nervous systems could create the sensation of pain, pain would not exist.
The common sense view, of course, is that most of the world is objectively real. It exists independently of whether or not anyone is experiencing it. The moon is there whether anyone sees it or not. This common sense view is called realism.
Realism is the view that there is a world outside our minds existing independently of our minds and perceptions, whereas idealism holds that the world is created by our minds by means of the observations we make. Idealism is a philosophical expression of the ideas popularized by the movie The Matrix.
Idealism strikes most of us as at best counter-intuitive. We're accustomed to think of matter as the fundamental reality (a view called materialism). Matter, we assume, is objectively real and exists whether we perceive it or not. On this view, whatever mind is it's somehow a creation or function of our material brains. Idealism turns this view on its head and declares that mind is actually the fundamental reality and that matter only exists as a subjective experience in our minds.
As I said, this view is counter-intuitive, but it's the view held by a lot of physicists who study the fundamental quantum structure of the world. This video gives a pretty clear idea of the thinking of many physicists, some of whom think that idealism is not only correct but that it leads to the conclusion that there is a God, or something very much like God.
The video's a bit long (17 minutes) and moves quickly. It also discusses some arcane physics at points along the way. Nevertheless, you don't have to understand the physics in order to follow the narrative. The science really only illustrates the basic idea which is that mind is fundamental and that matter is downstream, as it were, from mind.
Give it a click, kick back and savor how mysterious is the world in which we live and move and have our being: