Smoothing increases the number of facets in the mesh, resulting in a rounder object.

Facets are the underlying components of each mesh face. You can increase the smoothness
level up to the value of SMOOTHMESHMAXLEV as long as the number of faces does not
exceed the value in the SMOOTHMESHMAXFACE system variable. If you select multiple
objects with differing levels of smoothness, their respective levels are increased
by one.

You can only smooth mesh objects. However, you have the option of converting some
types of objects to mesh during the smoothing operation. You can also filter out ineligible
objects that you do not want to convert. For a list of objects that can be converted,
see MESHSMOOTH.