The Glided Age was the age after the Reconstruction where wealth and economic growth had sky rocketed, mostly in the United States.
When an industry got monopolized by one huge company, this meant that there were little to no competition for other companies against the monopolized company.
The company was making a lot of money and did not need to rely on workers as much, so a lot of the times would start paying them less. The workers couldn't boycott or threaten to leave, because the company wouldn't care if they lost some workers.
A lot of the time workers were forced to work very unsafe and unsanitary jobs for less money due to certain industries being taken over by companies.