"To Helen" is riddled with allusions to Greek and Roman mythology. Those allusions suggest how utterly blown away the speaker feels by his beloved's beauty: he can only begin to describe her in terms of the goddesses and legends of the past.
The poem's classical allusions start in the very first line, where the speaker addresses his beloved as "Helen." That name is a reference to none other than Helen of Troy, the gorgeous Greek princess who often gets blamed for kicking off the bloody 10-year Trojan War (in spite of the fact that the blame might more rightfully be laid on the various princes and kings who squabbled over her). If the speaker's beloved is "Helen," then she's the picture of beauty itself, a goddess among women.
The speaker drives that idea home when he imagines his "Helen" as "Psyche" herself in the final stanza. Psyche was the goddess of the soul, but she began her life as a breathtaking mortal princess, so beautiful that Eros, the god of love himself, fell in love with her. (She eventually earned her goddesshood after undergoing a series of dreadful trials.) As "Psyche," the speaker's beloved seems like more than a mere beauty: she's a real soulmate, a figure who touches him in the deepest part of his heart.
The poem's references to the beloved's "Naiad airs" (i.e., her nymph-like charms) and her "classic face" (like the perfectly proportioned face of a classical statue) underline the idea that her beauty is both powerful and timeless. The speaker is here imagining a figure of idealized loveliness, of the sort that many 19th-century artists and thinkers associated with the "glory" and "grandeur" of ancient Greece and Rome.
And the speaker thinks of himself in classical terms, too. He imagines himself in the role of Odysseus, the famous Greek hero who was cursed to wander the seas for years after he angered the sea god Poseidon (those are his "barks," or ships, in line 2). This allusion gets at the speaker's deep alienation—and the way that the sight of his beloved Helen seems to cure that alienation. This speaker doesn't ever seem to have quite felt at home in the world, but when he sees the beautiful face of his beloved, he feels like Odysseus did when he finally crawled back onto the "native shore" of his homeland.