An index is an estimate of the value of several different properties (such as stocks, bonds, art, commodities, etc.) taken together. In other words, an index is a bunch of different items that are combined and assigned a monetary value. For example, the S&P 500 is an index that tracks 500 individual stocks and translates their performance into one number. So instead of you tracking all 500 of those stocks, you can just look at the S&P 500 to get the gist.