The speaker is a parent addressing a baby, and the poem is really a string of names piled one on top of another. The child is a "candlelight," a "keeper," a "rebellion in the heart," "frantic blood." None of these labels settles into a single picture, and that seems to be the point. The speaker can't find one word big enough, so they keep reaching for another.
The images pull in different directions. Candlelight is small and warm, while "rebellion" and "frantic blood" are restless and fierce. "Brave new world" suggests both hope and the fear of an unknown future. The line about "living only on woman's milk" brings everything back down to the baby's helplessness, how tiny and dependent the child is despite all these grand titles.
The last phrase, "cipher of a child," suggests a puzzle that can't yet be read, and perhaps also a zero, a blank. The mood is tender, awestruck and a little dizzy, the feeling of loving someone you don't yet know.