The poem looks at a new mother outdoors in the morning and finds a kind of glow in her. The title, "Radiance," points to that glow, but the poem quickly makes it physical. Her "light" isn't just a pretty effect of the weather. It is her life force, and it extends "as far as the birth of her child / as far as the milk." The repeated "as far as" feels like someone tracing a boundary, then realizing the boundary is enormous: birth, and then the ongoing work of feeding.
The speaker seems quietly awed rather than sentimental. The language is plain and the lines are short, so the poem moves slowly, almost like careful observation. Judging by the imagery, the mood is tender and hushed, with a sense of wonder at how a body can become a source of nourishment. Ending on "milk" instead of something grander keeps the radiance earthy and real.