In the volume formula $\frac{1}{3}$*π*$h\ast (R^2+r^2+R\ast r)$, what does the term $R\ast r$ represent geometrically?

The $R\ast r$ term arises when subtracting the small cone's volume from the big cone's; it is related to the geometric mean $\sqrt{pi\ast R^2\ast pi\ast r^2}$ of the two end areas.