Themes contain a number of unique segments (buckets), which are shaded differently. You can see these if you look at the legend.
Sometimes a segment can relate to one certain value - like a Mosaic Group. Usually these are non-numeric.
Otherwise a segment can cover a range of numeric values (Ranged Themes). This is the case with the example shown.
Typically 'Ranged Themes' contain ranges of values of equal size between zero and the maximum value in the dataset.