Window is the way you look at the world. When there is a curtain, there is a veil between you and what lies beyond the window. When you are looking at the rays of the sun directly, it hurts your vehicle to do so. The magnitude of the light vibration and its energy are blinding. This is how The Goddess or the Master Teachers might appear to us without a filter.
Yet through the curtain you are able to glimpse the light that streams toward you from the other side. This is light your eyes can use and adjust to using. This light can reveal that which exists on the other side of the window frame. It can impress images onto the curtain--shadows, if you will. They are not "dark" shadows, but the shadows of things that live and exist in the light that emanates from the other side.
This is the principle behind spiritualist philosophy. Truly there is life that exists on the other side of the window frame. That life is just as real, or more real as our earthly lives. That life is, as we are, in need of light source vibrations to live as we live.
The window is a border -- a frame -- that we can use as a focusing lens.
In your meditations, you might want to use the window "frame" as a visualization tool. It will help you focus on the dualities of whatever states of being you are contemplating. For example, you might wonder "What about that thing, idea, quality over there in the subconscious (dimly visible) exists also on this conscious side (more visible)? What shadow of a reality are you seeing? How does it, in a supraconscious way, respond in its truest Self in relation to the shadow and light?
Definition: Self is a quality of being. Self is the allness of you. Mostly you see the material dense self--with a little 's'; that is, the You that you equate with the vehicle. This other Self exists beyond and in addition to what you know as self.
You see, you are the leaf -- both as shadow, and leaf, and idea. Even at that, you have yet to see all that you truly are. Rejoice in your complexities and quiver in the gentle wind and light.
For another take on truth ane light, See Plato's Allegory of the Cave.