Social Darwinism is the belief that people and business are subject to the same laws of nature and natural selection. This has been used by business leaders, both in the Gilded Age and today, to create a belief that the strongest businesses are the ones that survive because they survive the competitive process of natural selection.
So, massive oil companies and railroad companies (and tech companies and banks today) used social darwinism to explain their rise to almost monopoly status as a "natural" thing that was accomplished merely using the laws of nature.