Subdivision Surface
A modifier that rounds and refines a mesh by subdividing it.
The Subdivision Surface modifier splits every face and smooths the result, turning a simple cage into a dense, rounded surface. It rewards clean quad topology and tight loop placement, and punishes n-gons and stray poles. It is one common path to smooth models, not the only one; hard-surface work often relies on bevels and shading tricks instead.
Related: Loop Cut · Quad · Modifier Stack
These fundamentals are covered hands-on in Blender Geometry Essentials 01, free with a free account.