By "plutocracy," Americans meant a government and society controlled by the wealthy.
This famous cartoon by Horace Taylor, who was a fierce Democratic partisan in the 1900 presidential campaign, showed the oil magnate, John D. Rockefeller, who controlled most of the oil refining of the nation through his Standard Oil Company, holding the government of the United States in the palm of his hand.
Horace Taylor also believed that the great trusts had captured the government under the Republicans. As a Democrat, he of course argued that "the people" would regain control by choosing his party. In the meantime, under the Republicans, ordinary Americans suffered from the power of the trusts.