S&P/Case-Shiller Home Price Indexes

What Does S&P/Case-Shiller Home Price Indexes Mean?
A group of indexes that tracks changes in home prices throughout the Untied States. The indexes are based on a constant level of data on properties that have undergone at least two arm's length transactions. Case-Shiller produces indexes representing certain metropolitan statistical areas (MSA) as well as a national index.   

Investopedia explains S&P/Case-Shiller Home Price Indexes
S&P/Case-Shiller Home Price indexes are used as the underlying pricing mechanism in CME housing futures and options. CME housing futures and options trade on different indexes representing 10 different metropolitan statistical areas and a composite index representing 20 metropolitan statistical areas.

This is not to say that the indexes are a perfect representation of the housing market, because only single-family dwellings are included in the calculation. Furthermore, because some of the metropolitan areas are so large (Such as New York City or Los Angelas), having just one value may not accurately represent all areas within that city.