What does Alfred, Lord Tennyson mean by "the region of shadows" and the region of "realities"?
Tennyson says "The newborn love for something, for someone in the wide world from which she had been so long secluded, takes her out of the region of shadows into that of realities."
Concerning Tennyson's regions of shadows and of realities in "The Lady of Shalott," I'll just pick up where the first answer finishes. The poem is thought to be a treatment of the world of the artist and the world of the actual;illusion and reality. The Lady lives exclusively in the world of illusion, of art, of pictures and weaving. Outside of her tower is the actual, reality, life.
Depending on how allegorical one wants to be in interpreting the poem, Tennyson may be commenting on artists who isolate themselves and lose touch with the world outside of their imaginations. He seems to be suggesting that artists must continue to live as well as create art. Imagination was still central to art in Tennyson's period, but it should not be engaged in at the cost of all else.
The words that you are citing here are ones that Tennyson is said to have written (or somehow told his son) to explain what he is trying to talk about in this poem. So you should look at how the words apply to the situation in the poem.
For most of the poem, Lady Shalott is living in the region of shadows. That is because she can only see "shadows" or reflections of the world. She is not able to fully see reality or to participate in it.
However, once she sees Lancelot, she has an uncontrollable urge to go into the region of realities. In the poem, this means that she stops looking through the mirror and looks out (and later goes out) into the real world.
I think the region of shadows literally means the mirror -- her only way of perceiving the world at first. The region of realities is the real world that she later looks at. Metaphorically, you can argue that the region of shadows is a fantasy world that is just inside one's mind.