In general, you adjust normals and smoothing to prepare objects for rendering.

A normal is a unit vector that defines which way a face or vertex is pointing. The direction in which the normal points represents the front, or outer surface of the face or vertex, which is the side of the surface that is normally displayed and rendered.

You can manually flip or unify face normals to fix surface errors caused by modeling operations or by importing meshes from other programs.

Smoothing groups define whether a surface is rendered with sharp edges or smooth surfaces. Smoothing groups are numbers assigned to the faces of an object. Each face can carry any number of smoothing groups up to the maximum of 32. If two faces share an edge and share the same smoothing group, they render as a smooth surface. If they don’t share the same smoothing group, the edge between them renders as a corner. You can manually change or animate smoothing group assignment. Changing smoothing groups does not alter geometry in any way; it only changes the way faces and edges are shaded.