See Subdivision Surfaces for more information on subdivision surfaces.
When rendering smooth surfaces from polygons in Houdini, you can add an option to render Polygons as Subdivision to your geometry node from the Edit Rendering Parameters...
Add ri_rendersubd from the Render Properties to your object's Render TAB.
The images below were rendered using the PxrVisualizer integrator in Houdini.
We also need to add the interpolate boundary and turn it on. It is off by default.