This a period of American history at the end of the 19th century. Opinions differ on when it started – certainly post-Civil War, but various accounts have it starting in the 1860s or 1870s and running through the end of the century.
The period was characterized by enormous post-bellum economic growth accompanied by incredible urban poverty.
The name is a pejorative joke. “Gilding” is a technique of putting a thin layer of gold over a cheaper substance. So, rather than a “Golden Age” where the entire of society benefited, we had a “Gilded Age” where a small part of society benefited and that was a decorative layer over larger societal problems.
The age is associated with the rise of barons like John D. Rockefeller, Andrew Carnegie, and J.P. Morgan, and with the construction of large homes which have since become historic sites.