How did social darwinism explain how society operates?

Social Darwinism explained the operation of society as "survival of the fittest."

People often associate the term "survival of the fittest" with Charles Darwin, but actually it was terminology that came from Herbert Spencer, the founder of what is known as Social Darwinism. (Spencer wouldn't have called it that - he and Darwin were rivals.)   Darwin used the term "natural selection" in his descriptions of biological evolution.  Spencer was unafraid to apply the same concepts to competition within the human species, contending that the stronger, fitter social or cultural groups will survive better than others and dominate over weaker groups.