Main street is a colloquial term used to refer to individual investors, employees and the overall economy. "Main Street" is typically contrasted with "Wall Street." The latter refers to the financial markets, major financial institutions and big corporations, as well as the high-level employees, managers and executives of those firms. You'll often hear about Main Street vs. Wall Street in rhetoric about the differing goals, knowledge levels, interests and political power of these two groups. Some people think that what's good for one group is bad for the other. For example, high executive pay is seen to conflict with ordinary workers' pay and job security.
Main Street can also describe a small, independent investment company (i.e., a Main Street firm) as opposed to one of the large, globally recognized Wall Street investment firms. Wall Street firms tend to serve large investors with multi-million dollar assets, like institutions, while Main Street firms tend to be better suited to serving small, individual investors by providing more personalized service.