Retopology
Retopology is the act of recreating an existing surface with more optimal geometry that is a clean and quad-based mesh. It is the process of converting high-resolution models into something much smaller and into simplified versions that can be used for animation.
The artistically-driven nature of modern 3D sculpting allows for a fast workflow and stunning results, but the sculpted mesh is often messy, cluttered, tangled and unsuitable for animating or applying textures. The process of sculpting in tools like ZBrush, Mudbox or Blender results in high resolution models with many unnecessary polygons. While a 3D artist would build a mesh polygon-by-polygon (box modeling), digital sculpting is a more intuitive process that lends itself better to how artists think. The downside is that you have to create a lot of polygons to sculpt the curves you need. And too much detail will slow down even the most powerful computer. That’s where retopology comes in. The main use of retopology is to get a polygon mesh at a smaller file size that’s useable for animation. Complex meshes are difficult to animate. It is necessary to limit the polygon count of any 3D model when it’s used for animation. Through retopology, you recover a more efficient 3D surface that’s better for texturing and animating either for animated film or video games. The surface of a character would be approximated using the simplest forms possible. Later more details could be added, but the original mesh would be preserved for animation. The bottom line is that in this way you can preserve the details you get from high-resolution work, but still generate a model that runs smoothly with animation.