The Eldorado Exploring Expedition is a group of five white men who come downriver to seek treasure, one of whom is the Station Manager's uncle. Their purpose is to find treasure and exploit African resources, mostly ivory. To Marlow, they seem uncouth and uncivilized, without necessary knowledge or compassion.
This devoted band called itself the Eldorado Exploring Expedition, and I believe they were sworn to secrecy. Their talk, however, was the talk of sordid buccaneers: it was reckless without hardihood, greedy without audacity, and cruel without courage; there was not an atom of foresight or of serious intention in the whole batch of them, and they did not seem aware these things are wanted for the work of the world.
(Conrad, Heart of Darkness, eNotes eText)
While Marlow still has his idealism, and has a specific mission in Africa to find Kurtz, these people are nothing more than treasure hunters, willing to kill natives without concern and damage the environment to get what they want. Marlow realizes that these are the rule in Africa, rather than the exception; while he is working overtime to get the steamboat ready for its journey, they have no "serious intention," meaning that they expect others to work for them instead of striving for their own goals. Marlow sees them as the worst facet of European expansion into Africa, because their selfish motivations will bring harm instead of benefit.
Well, Eldorado was a mythical city of gold, a place for people to get rich quickly. Even though it isn't a real place, many people were willing to risk their lives to find this gold.
While there is no gold in Africa, there was ivory. Ivory at the time was a huge commodity and a big way to make a lot of money. The white people were risking their lives to get this because many of them were dying in the Congo. If they don't die, it still has a very negative effect on them (ie Kurtz going crazy with wanting so much of it).
It also highlights the imperialistic attitudes of the explorers. They are willing to harm and kill the Africans just to get these riches.