According to the page on Yahoo Finance,
the volume of S&P 500 is 1,813,371,531. What is the meaning of this "volume"?
It should be the total volume (daily shares traded) of the component stocks.
Warning: Yahoo shows a different (much larger) volume number in S&P 500 historical quotes and charts (and also in the "Avg. Volume" field). This is actually the NYSE Composite volume, as is seen because identical numbers appear in historical quotes and charts for ^NYA.