The graph plots the log fold change or the change in the size of counts between Meristem and Leaf samples versus the mean of normalized counts or the mean of counts for sequences in the samples.
Data shows that to the left of the graph at low mean of counts there is very low fold change or change in abundance of sequences Moving to the right on the x-axis as the mean of counts increases, the sequences that are more abundant are above the zero line and sequences that are less abundant are below the zero line as seen by blue data points away from the zero fold change line
Thus sequences with low mean of normalized counts are equally abundant in the samples.