Eldorado (or, El Dorado): that's the name the Spanish conquistadors of the 1500s gave to a legendary city made all of gold. Based on conversations with South American natives and the precious gems that some of them were wearing, the Spaniards concluded that there was some gilded, or golden (this is what "dorado" means), city somewhere out there. Well, the joke was on them, as it turns out. There wasn't any city of gold, or at least there wasn't one that anybody ever found. Like, ever.
Poe wrote "Eldorado" in 1849 as a response to the California Gold Rush. The fact that his immediate response to an event that gripped the nation was to talk about Eldorado suggests that he saw the rush for gold as a quest for something unreal, as a pursuit that could only end in disappointment (which, to be fair, it did for many).
But Poe's poem isn't really about the Gold Rush, per se, though it is an oblique (indirect) comment on it. It's more about being consumed by a search for something whose grip is so powerful that it comes to be just like Eldorado was for the Spanish, like Californian gold was for many Americans. The knight in the poem spends his whole life searching, never finds anything, and eventually dies (or at least is really close to dying). Eldorado is more than the title, then. It's Poe's word for a hope or dream so powerful that it causes one to forget about pretty much everything else in life. By the poem's end, the knight is alone, he doesn't really have anything, and he dies (or is about to die) with nothing except a vision of Eldorado in his head. And that, gang, is not too pleasant.