F3 statistics is used to detect the presence of admixture in a population (X) from two other populations (A and B).

The value f3(X; A, B) is negative when X does not appear to form a simple phylogenic tree with A and B, but rather appears to be a mixture of A and B. Thus, the more negative the f3 statistic, the greater the implicit admixture.

A negative value implies that X is an admixture of A and B, rather than forming a simple phylogenic tree. 

So in this chart, it implies trees C, D, E and F (they have admixture), and not A and B (which are simple trees showing no admixture)



So if we are looking at this figure it is testing for f3 (X, Y; Korean) and f3 (X, Y; Japanese):


A score further from 0 reveals a "stronger signal", i.e. producing a greater effect when detecting the presence of admixture in a given population.

Thus:
(1) From the samples, admixture of Devils Gate2 and Ami are closest to modern Koreans (not Nganasan and Atayal)
(2) From the samples, admixture of Devils Gate2 and Ami are closest to modern Japanese

NB: all of the plots shown are statistically significant, they did not include those that were insignificant.