The idea of Manifest Destiny was a philosophy which led to many people believe that the US was "destined" by God to expand across North America. In other words, it was meant to create several new states, to expand its dominion and spread democracy and capitalism across the entire North American continent.
These new states had to decide whether to be slave or free, which gave rise to disputes that couldn't be solved without the Civil War.
There are three basic themes to manifest destiny:
The special virtues of the American people and their institutions
The goal of the United States to redeem and remake the west in the image of agrarian America
The destiny to fulfill this fundamental duty