Shape of the curve when the majority of the scores fall at one end of the distribution, with the remainder of the scores tapering off as reflected in the long tail of the distribution
This is a reference to the shape of a histogram of tree lengths that can be produced after searching through treespace. Studies of random datasets have shown that the distribution of tree lengths is approximately normal, whereas in general datasets with a reasonable amount of signal have few shortest trees and few trees nearly as short. There is a G-statistic for the skewness of a histrogram.
Skewness characterises the degree of asymmetry of a distribution around its mean. Positive skewness indicates a distribution with an asymmetric tail extending towards more positive values. Negative skewness indicates a distribution with an asymmetric tail extending towards more negative values.