Corrects R2 to account for the degrees of freedom in the data. In other words, the more data points you have, the more universal the regression equation is. However, if you have only the same number of data points as variables, the R2 might appear deceptively high. This statistic corrects for that.
For example, the R2 for one equation might be very high, indicating that the equation accounted for almost all the error in the data. However, this value might be inflated if the number of data points was insufficient to calculate a universal regression equation.