The poem captures a small, ordinary moment: someone walks indoors from bright sunshine and can't see anything. The first line, "no shadow is cast," is a little puzzling. It might mean the sun is directly overhead, at its brightest, so that nothing darkens the ground beneath the speaker. Judging by the wording, it could also hint at a feeling of being weightless or unattached, as if the speaker leaves no mark on the world.
Then comes the shift. Stepping inside, the speaker finds the house dark. Anyone who has gone from glaring light into a shady room knows this effect, when the eyes need a moment to adjust. But the contrast also gives the poem a quieter feeling. The outside is so full of light that it has no shadows, while the inside seems almost empty of it.
The mood is still, a bit lonely, and thoughtful. The speaker doesn't say how they feel, so we sense it through the sudden change from brightness to dimness.