Three short lines, and the whole poem is a moment of noticing. The title, "Earth," sets the scale, and then the poem shrinks it down to something small and specific: evening light falling among the corn. The speaker calls it "a rare world," as if stepping into that light means stepping into a place that doesn't come around often.
The word "rare" does a lot of work. It suggests the speaker feels lucky to be there at just the right time, when the light turns soft and the ordinary field becomes something special. Nothing dramatic happens. There is no action, only looking, and that stillness is part of the feeling.
The mood is quiet, calm, and a little awed. The poem is also open-ended. It doesn't tell us exactly what the light looks like or how the speaker responds, so we fill in the glow ourselves. It reads like a held breath, a pause to appreciate a beauty that will soon fade with the day.