This is the ratio of the number of transitions to the number of transversions for a pair of sequences. R becomes 0.5 when there is no bias towards either transitional or transversional substitution because, when the two kinds of substitution are equally probable, there are twice as many possible transversions as transitions. MEGA allows you to conduct an analysis of your data with a specified value of R.
Note that R should not be confused with the ratio of the transition and transversion rates (k = a/b).