What do you think John O'Sullivan means by "our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions"?

That the United States destiny is to spread and expand in order to have enough room for our growing population

That it is the United States' destiny to expand across to the pacific to expand and spread religion, resources, start civilizations. Many of the settlers believed God wanted them to move across the west and that it was inevitable.