The JSE index is one measure of the financial health of the South African nation, 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 battle for the presidency between Ramaphosa and Zuma will have a strong influence on the index during 2018. World economic conditions have an influence. Confidence in corporate South Africa, after shocks like Steinhoff and the Guptas, will also affect the price.