What does Webster mean when he says the strength of america will be in the valley of the mississippi?

This is one of the great compromises of his time. There were territories asking for statehood. The problem being talked about is slavery. America needed to have control of the mississippi for food and trade. To have this "trade strength" he would later on suggest that two of the four states be free and the others, slave. By declaring them states, they would belong to America. But the strength was about trade of food and then cotton and tobacco.