The months' supply is the ratio of houses for sale to houses sold. This statistic provides an indication of the size of the for sale inventory in relation to the number of houses currently being sold. The months' supply indicates how long the current for sale inventory would last given the current sales rate if no additional new houses were built.
Source: US. Bureau of the Census
Release: New Residential Sales
US. Bureau of the Census, Monthly Supply of Houses in the United States [MSACSR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/MSACSR/, November 26, 2015.