The speaker of "You're" is a pregnant mom affectionately describing her unborn baby. The title seems to start a sentence or pose a riddle: "You're"—what? Each of the sentence fragments in the poem provides a possible answer, an attempt to describe this brand-new person through figurative language.
In these first two and a half lines, the speaker reels off two similes and a metaphor. The first simile compares the baby to an acrobatic "Clown[]" who is "happiest" in a tumbling position: "on your hands, / Feet to the stars." This is an imaginative description of the head-down, feet-up position babies often take in the womb (especially in the later stages of pregnancy). The second simile, "Gilled like a fish," refers to throat structures called gill arches that human embryos acquire during their development in the womb. In fish, similar structures develop into gills; in humans, they develop into parts of the jaw, larynx, and ear. In between these two similes, the metaphor "moon-skulled" compares the baby's pale, bare, still-developing skull to a pale, bare (perhaps not quite full) moon.
Here and throughout the poem, then, the speaker reaches for vivid, imaginative comparisons to evoke the strange new life growing inside her. These opening lines also establish the poem's lack of a meter, and it has no rhyme scheme, either; it's written in free verse. (However, each stanza consists of nine lines of roughly even length, mirroring the nine months of a typical pregnancy.) Already, readers can hear the ear-pleasing sound effects that will continue throughout this playful poem: alliteration ("happiest"/"hands"), consonance ("moon-skulled"/"Gilled"), and assonance ("Gilled"/"fish").