Specular highlight
The small bright spot where light bounces straight off a shiny surface into your eye.
See it in WebGL playground. Lower Roughness under Material and watch a sharp highlight appear.
In plain words
Shiny things show a small, bright spot of light. That spot is where light hits the surface and bounces straight toward your eye, like a mirror. It moves when you move, because the angle changes. Smooth surfaces give a small, sharp spot, and rougher ones give a wide, soft glow. Without highlights, 3D shapes look flat and chalky.