The late 19th century is called "the Gilded Age" because it was successful and wealthy on the surface but corrupt if examined more closely. The term was coined by Mark Twain, who wrote a novel with that title, which satirized powerful leaders of the era.
Gilding is a process where a thin layer of gold is laid over a less valuable material to give the appearance of gold without the expense. This related to the late 19th century in the United States because it was the era of robber barons, or wealthy businessmen who grew even more wealthy due to unethical business practices. They often exploited laborers and strove to create monopolies. Many entered into trusts with other businessmen so they could artificially drive up costs and then split the profits.
However, the Gilded Age was also important because the widespread corruption led to significant reforms. The labor movement fought for workers' rights, and many laws that regulate shady businesses such as trusts and monopolies were passed during that time period. Although most of this process was peaceful, sometimes strikes and other union protests became violent. Reforms were often supported by the middle class, who noted the dangerous conditions that most factory workers and immigrants lived with and wanted to help.