What is it ?
A Visual Effect Compositor (or Digital Compositor) is the last artist of a long chain to work on a shot. His role is to gather all computer generated (CG) assets from others departments and compose them into the final footage, like an advanced patchwork. His skills are as much technical like green screen extraction, tracking or retiming elements as artistic with a great knowledge of light, color and composition. Such artist is able to modify any element and integrate them in a picture with harmony and precision to create a believable and photographic picture.
On a photographic level, a VFX compositor must know how the light (in the physical world) is reacting in a scene and on materials (wood, metal, skin...). A lot of physics interact such as reflection, refraction, transparency or subsurface scattering.
Why is the sky blue ? What blue ? How to reproduce that blue ?
Does the light react the same on wood and metal ?
Then the artist reproduces the effects from the camera like grain, lens distortion, bloom or lens flare. The camera deforms the picture and the spectator has unconsciously integrated these transformations as "this is how a movie looks like", the VFX compositor have to replicate all of these.