An index measures the total worth of some class of stocks.  (Here, the worth of a stock can be defined as the current price of a share times the number of shares in existence.)  For example, the S&P500 is an index of 500 of the largest U.S. companies.  It includes about three fourths of the entire U.S. stock market.  Note, the index is proportional to the total worth of the class of stocks.  The actual worth of the whole class of companies would be billions of dollars.  The index is a smaller, more manageable number.  It is not measured in dollars, and the numerical values of different indexes cannot be compared to each other.  However, a change of one index over time gives meaningful information about that one class of stocks.

Each index refers to a particular class of stocks.  An example that might interest green investors is an ESG leaders' index.  For this, one excludes any class of companies that is considered totally unacceptable, such as companies that own, produce, or transport fossil fuels.  The rest of the market is partitioned into about 10 sectors, such as communication services, consumer staples, financials, information technology, etc.  Each company is assigned an ESG score (a measure of Environmental, Social, and Governance goodness), and in each sector the companies with the best ESG scores are kept.  The resulting class gives an ESG leaders index.