FII stands for Foreign Institutional Investor, the older name for what SEBI now calls Foreign Portfolio Investors, or FPIs. These are overseas pension funds, sovereign wealth funds, hedge funds and asset managers that invest in Indian shares and bonds. Because they move very large sums and react to global interest rates, the dollar and risk appetite, their buying and selling can drive the direction of Nifty for weeks at a time. NSE publishes their daily net figures. A month of FII selling worth tens of thousands of crores usually coincides with a weak rupee and a falling market, while sustained inflows often accompany rallies. DIIs frequently take the opposite side. For a retail trader, FII data is background information that explains why the market is behaving a certain way; it is not a signal to copy, since their positions, hedges and time horizons are not visible.