Answer:
Manifest Destiny was the idea that Americans, that is to say, the United States, had the right to expand to the West, and reach the Pacific Ocean: occupy the lands in between, and spread American values.
It also applies to other parts of the world in a more indirect way, especially Latin America.
The settlement of the West was aided both by economic necessity, because many migrants were arriving in the United States seeking for cheap land, and also by ideology, and the ideology was precisely that of Manifest Destiny.
