In economics and finance an index (for example a price index, a stockmarket index) is a benchmark of activity, performance or any evolution in general.
Consumer price indexes (an inflation measurement,) or also a country GDP index (an economic growth measurement) can be used, among other things to adjust salaries, bonds interest rates, tax thresholds...
Also, some investment funds (index funds manage their portfolio so that their evolution always mirror (tracking) the evolution of a stockmarket index.