What is Subdivision surface?

Instead of modelling every curve directly, artists model a simple, blocky cage and let the computer smooth it. Each level of subdivision splits every face into smaller ones and moves the points toward a smooth average. The most common algorithm is Catmull-Clark, which Blender's Subdivision Surface modifier uses.

Controlling the smoothing

Left alone, everything turns blobby. Artists add support loops (extra edges near a corner) or crease values to keep edges sharp where needed. This is why quad topology matters: triangles and n-gons create visible pinches when subdivided.

Cost

Each subdivision level roughly quadruples the face count. Two levels turn 1,000 faces into about 16,000. For real-time use, apply a low level or bake the smoothness into a normal map.

Subdivision and 3D printing

Low-poly generated models print with visible facets. A level or two of subdivision before exporting to STL can make figurines look smooth, though it also softens edges you wanted sharp.