The poem begins with the speaker describing his first glimpse of a beautiful woman. "She was a Phantom of delight," he says, a metaphor that compares the woman to some kind of enchanting spirit. That she "gleamed upon [the speaker's] sight," meanwhile, suggests that she seems to flash or shine before him, a radiant creature whose otherworldly radiance has caught his eye. Together, these opening lines suggest that she's captivating to behold, but also that she's somehow not quite real.
Indeed, the speaker next calls her a "lovely Apparition," or ghost, who was "sent" (to the speaker, or perhaps down to earth) "To be a moment's ornament." She's a kind of decoration, an object making the moment lovelier—not a full human being.
The poem is written in iambic tetrameter, meaning that each line is composed of four iambs (feet made up of an unstressed syllable followed by a stressed syllable). This meter gives the poem a bouncy, steady rhythm. The poem also follows a very straightforward couplet rhyme scheme, where every two lines end in a rhyme ("delight" and "sight," "sent" and "ornament," etc.). These rhymes add to the overall musicality of the poem while also emphasizing key words and ideas (i.e., the "delight" the speaker felt the first time he saw this woman).
Finally, notice all the /n/, /m/, and /t/ consonance and /ent/ assonance/consonance in these lines ("Phantom of delight," "gleamed upon my sight," "sent / to be a moment's ornament"). These sounds add musicality, evoking the woman's ethereal beauty and the intensity the speaker feels upon seeing her for the first time.