This is the average everyday way of thinking about things--as if they aren't in your head. It's also the way that someone like Aristotle thought about things. The ancient Greek philosopher thought that things were composed of form and matter. When you thought about a thing, you thought about its form. The matter of a thing, it's body, was the stuff that didn't get into your head. The dual form-matter composition of things meant that things could be both inside and outside your head, at the same time. An idealist, however, wants to points out that the thought a person has about a tree and the tree itself are actually the same thing: a thought about a tree.
Now how does that follow? Notice how when you distinguished between the thought about a tree and the tree itself, it was you who distinguished between the two. You never stopped thinking. You thought about the thought about a tree, and you thought about the tree itself. And lo, you discover the tree itself was actually your thought about a tree. (Try not to think about this too hard. Just accept it as true. It'll save you a lot of headache and pain.)
The idealist seems to suggest that the tree is nothing more than a thought in your head. That seems to be the consequence. Well, yes and no. Yes, on the basis of the point made above, that the thought a person has about a tree and the tree itself are actually the same thing: a thought about a tree. But no, because even the idealist recognizes that the reason we feel compelled to distinguish between thoughts about things and the things themselves must be given some explanation.
Part of the problem, the idealist thinks, has to do with what is meant by 'your head'. While different idealists employ different sets of terms, a idealists generally tend to identify two sorts of consciousness, one finite and the other infinite. Finite consciousness, the one that draws distinctions between thoughts and things, is 'the head' in which people feel most comfortable. Infinite consciousness, on the other hand, embraces all finite consciousnesses--yours, mine, his, hers, ours, theirs, etc.
At the very base, the idealist insists, every finite consciousness is merely a manifestation of infinite consciousness, in which thoughts about things and the things themselves are both the same thing: thoughts about things. On what basis they do is not always clear, but let's overlook this for a moment. Finite consciousnesses, like our own, may understand that thoughts and things ought to be the same thing: thoughts about things. Only an infinite consciousness, however, actually understands what it is for thoughts about things and the things themselves to be the same thing.
And that is what the idealist means when they say they want to think God's thoughts after him.