Social Darwinism was a prominent intellectual theory that was used to justify European Imperialism in the 19th and early 20th centuries.
Social Darwinism held that those made better or worse by industrialization were based on the natural predisposition of the people effected. For those who were made better off, the success of those people demonstrated that they had a well-evolved culture and this expansion would help remove or improve underperforming cultures, which were those "chosen" by the fact that they were performing poorly.
Imperialism may encompass a broad range of meanings, including the foreign occupation of one nation by another; one society's control of the internal politics of another; and, especially, foreign extraction of a less powerful nation's natural resources and/or exploitation of its indigenous human resources.
All in all, according to the Social Darwinist, Imperialism was the natural expansion and success of a well-evolved culture and this expansion would help remove or improve underperforming cultures.