What is the Gilded age?

Gilded refers that which is golden, or appears to be golden---thus, in other words, the culture of money and its protection, so this answer emphasizes those aspects. In general terms, the wealth of the nation expanded 1870-1905, but the basis of severe economic inequalities were also starting to form in a new way. In part this was due to the enormous influx of foreign immigrants who usually lived under very poor economic conditions, and often brought freedom-demanding (or expecting) ideas with them from Europe. This was in obvious growing conflict with the upper classes who were closing off into an even more separate realm of wealth and polite manners to prove that the USA had finally achieved cultural (and economic) maturity on its own, although still bowing to European art and standards. It was also the time that the Supreme Court took on its unique role of protecting big business, and thus contributed to the ideology that to be free in America meant all were free, including the new legal idea of the corporation which in this view should be treated as a "free" individual, not as a company.

Also, although this was the age of Darwin, and many people adapted the idea of "survival of the fittest" to the business world, there were some wealthy people who believed somewhat that they were doing good for the whole of society by creating wealth that would enrich the nation, and all the people, and were thus even fulfilling a Christian duty in that way. (Some of these people of more modest middle-class means tried to implement some of the reforms of the Progressive era early in the 20th century, but they also tended to hesitate about the readiness of the "masses" to decide things for the nation as a whole.)

The effect for many years afterward was that for the extended period after, all the way even up to1935, or so, it was accepted that the Court would almost always approve private business interest over any government attempt to improve conditions. This included safe working conditions, one or two companies controlling an entire industry, social security, minimum wages, and many other controls on business that we take for granted now. The Courts during this time very frequently--usually-- said that the Government could pass no laws on these matters, even if they were national concerns, not just a particular state. (There were just a couple late court rulings in the very late 19th century that said the US could pass certain rather weak laws that were against the usual philosophy of letting business run itself by itself, if certain controls were clearly in the "greater public interest" than letting business do whatever it wanted, but this was rare.)

Thus, the two main effects of the Gilded Age were a stiffening of economic protection of big business and banking under the umbrella of legal protection, and the reasoning given as encouragement that nobody was hurt by this since anyone with ambition could become a rich man. (It must also be noted that following the Civil War, the problems of how to bring the South back into the national union was a difficulty. The strong moves of northern-led Reconstruction to quickly bring political and personal rights to the freed African-Americans was abandoned due to political compromises. Within twenty years after the war, the majority of ruling society had more or less agreed to let the South treat blacks as second class citizens, and let them pass the "Jim Crow" laws that said blacks and whites had to use separate facilities [such as schools, hotels and stores]. Again, the Supreme Court took the exclusionary side, saying in 1896 that such separation was legal, as long as the facilities were "equal." Of course, in fact they were almost never of the same quality for blacks, but this indicated the way society said in its words, perhaps even its partial beliefs, that the individual had rights and great freedom. But in reality very many did not, just as was true in the economic realm.)

Both beliefs were finally altered by the Great Depression when it was clear that the individual could NOT solve his own problems created by economic forces totally out of his control, could not depend on family or religious charity in the community, and secondly, that the Government not only could control and improve the life of the national community with its active interventions, but that perhaps it even had a social and moral obligation to do so. (And later the Courts, especially after 1937, when different sorts of judges were appointed, increasingly joined in the trend of protecting citizen rights, finding a balance between letting the market do whatever it naturally wanted, and the protection of the citizens on the other hand).

But it was, after the 30s, a revolution in thinking about business and the individual that has become thoroughly made part of the American majority of thought, even if we still argue about the ideas in its details from time to time. To some degree, the argument came up again during the Reagan era, when that President attacked the government using the old terms---that "government" was something that was not the People themselves, as the founders believed, but a force out to hurt the People and make trouble for business and free enterprise. It is now much less prevalent to hear such criticisms in the 21st century, since Reagan's rhetoric has faded somewhat from memory, and society had changed too much anyway to go back to his preferred ideas, so it was nothing like the extremes it was a century ago.

But the changes of the 1940s through the 1970s occurred very slowly since Gilded Age ideas of how economic expansion and social rights should be defined had been in control almost all of the late 19th and early 20th centuries, and the expansion of the idea of broader citizenship and basic social protections took a long time to take hold in both the legal and popular imagination, although that is mostly the case today.

Some would put it more simply: A time of growth and progressive reforms. Most of the growth was to an upper elite and large companies. Individual citizens did not gain many rights, and African-Americans lost many advances they had made after the civil war. Few progressive reforms were implemented, although some were discussed since the widespread corruption of the age, in politics and elsewhere, was openly admitted. Some would even say that it is called the "gilded" age instead of the "golden" age because the surface was shiny and improving, but many rotten problems were festering underneath the glittering paint of success shared mostly by a few, rather than the majority of citizens. (The Twain and Warner novel that gave the age its name---'The Gilded Age"-- is full of sardonic remarks about about the nature of the success values of the times, but many other publications shared their social concerns.)
The Gilded Age refers to the late 19th century between the 1870s and 1900. Mark Twain used the term "gilded" to refer to the appearance of the period being wonderful while underneath it was base and worthless in the same manner that gilded (gold plated) lead can appear to be gold but ut remains worthless.