Reflective 3D printing filaments utilise retro-reflective pigments mixed with a polymer to create a reflective material that can be 3D printed.
Retro-reflection is the term used to describe the phenomenon of light being reflected in the same or similar direction that it came from. Retro-reflective pigments are often metal-coated or glass beads that are commonly used in reflective clothing and traffic signs.
Some materials utilise diffuse reflection which scatters light in many directions. This has the advantage of making it extremely difficult for the human eye to differentiate between layers, effectively hiding layers without the need for post-processing.