Your mind (Greek, nous; German, Geist [often translated 'spirit', but always meaning mind]) is absolute because you are able to think about yourself and know that you exist: you don't need anything other than thinking to prove that you exist. When you think about the world, however, you only have the evidence of the senses to go on. You have to interpret sense perception. When you look at the desk you are sitting at, for instance, you can only understand that it is a desk because you have an idea of what a desk is to compare to what you see. This suggests that a desk exists in two different ways, as a concrete physical object, and as a concept of a desk that doesn't have any concrete attributes. The physical desk Hegel calls nature, while your conception of desks in general he calls an idea. Now, if nature or the concrete things we sense, define what being or existence is, then the ideal is not being. If on the other hand, the ideal defines what being is, then the desk in nature is not being. Hegel saw that these two possibilities contradict each other and called them a thesis and an antithesis. The synthesis, the thing that makes it possible for them to exist together, is the thinking that is going on in your mind, which is a rational process. Thus the part of all this that is real, is neither nature nor the ideal, but is the rational process of our thinking about the two different kinds of being. Thus what is real is rational and what is rational is real. What this means is that reality is identical with a person's rational understanding of reality. Something that is irrational (a floating castle, for instance), doesn't exist in reality. The nature of reality is structured according to human reason.
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