Like with elevation profiles, there are two ways to bring up a stats dialog. One is to right click on a line or polygon, and choose Stats. The second is to right-click on the map background, choose Measure, and then either Line Stats or Area Stats.
Either option will probably involve a short delay as CalTopo pulls up the relevant elevation and vegetation data, followed by a dialog. Unlike with elevation profiles, polygons will bring up data for the polygon's interior rather than just its perimeter.
The first two charts are elevation and slope histograms, along with min, max and average values; color-coding on the slope histogram matches CalTopo's slope shading layers. The third chart is a circular histogram tracking aspect across 45 degree slices. The pie slices are area-proportional rather than radius-proportional, i.e. if N aspects are 50% as frequent as W aspects, then the N pie slice will cover half the area of the W pie slice, with a radius that's 70% as large.
The tree cover histogram shows tree canopy coverage - 100% means that trees completely block all views of the sky. Land cover shows land coverage as listed in the National Land Cover Dataset (NLCD).