Normal

The direction a face points, which controls how light shades it.

Every face has a normal, the direction perpendicular to its surface, and shading is calculated against it. Flipped normals make faces render dark, invisible, or wrong in game engines. Blender can display normals as overlays and recalculate them (Shift+N) when a mesh's are inconsistent.

Related: Smooth Shading · Face

These fundamentals are covered hands-on in Blender Geometry Essentials 01, free with a free account.