Most valentines announce who they are for. Poe's does the opposite: it spends twenty lines insisting that a name is present while making it hard to find. The speaker tells a woman with "luminous eyes" that she "shall find her own sweet name" lying on the page, then dares every other reader to fail. The central idea is that a compliment can be a game of concealment, and that the most private form of praise is one only its recipient can unlock.
The key is a trick of position. Take the first letter of line one, the second letter of line two, the third of line three, and keep stepping down the page. The letters spell F-R-A-N-C-E-S S-A-R-G-E-N-T O-S-G-O-O-D, Frances Sargent Osgood. Because the name runs on a diagonal through the poem rather than down the left margin, it is buried inside the lines. That suits the instruction that the talisman "must be worn _at heart_": the letters really do sit at the heart of the lines, not at their edges. The "three eloquent words" that lie *perdus* (hidden) are, I'd infer, the three parts of her name, which the speaker says is "a poet's, too."
The poem describes its own hiding place as it goes. The name is "nestling," "enwrapped," tucked away like something small and precious. The speaker orders, "Search narrowly the lines!" and "Search well the measure," and "measure" works as a pun. In poetry it means metre, but here it also means the literal counting of letters that solves the puzzle. Even the warning that "you may lose your labor" over "the trivialest point" is a clue wrapped in a threat.
The classical and historical allusions add flourish. "The twins of Leda" recall Castor and Pollux, who became the stars of Gemini, so her eyes become a pair of bright stars. The Gordian knot, which Alexander famously cut with a sword, is brought in only to be waved away: this riddle needs no "sabre," just comprehension. The best joke is the "knight Pinto--Mendez Ferdinando." Fernão Mendes Pinto was a Portuguese traveller whose tales were so extravagant that his name became shorthand for liars. The poem says its letters are "lying / Like the knight Pinto." Here "lying" means both resting in position and telling falsehoods, yet together they "form a synonym for Truth." The name is hidden and apparently deceptive, but it stands for honesty, which is the real compliment.
The sound of the poem is playful, too. It runs in five ABAB quatrains, but the rhymes are stretched on purpose. "Leda" is paired with "reader," "labor" with "sabre," and "Ferdinando" with "can do." The lines are long, loosely iambic and often end on an unstressed syllable, so they sound like a showman's patter. Dashes and exclamation marks keep the voice breathless and teasing: "Search narrowly the lines!--" The final line sprawls out longer than all the others and ends on the deflated "you do the best you _can_ do," as if the speaker shrugs and walks off. "Cease trying!" is reverse psychology. The speaker forbids exactly what he wants you to do.
What makes the poem worth reading is that form and content are the same thing. It is a poem about a hidden name that actually hides one, and its instructions work as clues. It also splits its audience in two: ordinary readers are told they will fail, while the one reader it is written for is promised success. The poem makes reading into an act of intimacy, a love note that works like a lock with one intended key.
It also invites you to prove it for yourself. Take a pencil, step down the lines, and the name appears letter by letter. Poe wants you to feel the pleasure of solving it, even while he insists you can't.