The Dow Jones index is one measure of the financial health of the United States, at least the bit that buys shares. When it rises, investors, pension funds, and indirect share holders smile and expect more money in their pockets. When it goes down, dire warnings are issued. When it crashes, everyone panics.
The presidency of Donald Trump will have a strong influence on the index during 2018. World economic conditions have an influence. Confidence in corporate America will also affect the price.