Sit in darkened rooms, spin 3 trackballs, and turn a few knobs to make pictures look pretty, mostly. DaVinci Resolve[1] and Baselight[2] are quite popular, take a look at the websites to give yourself an idea of what it looks like.
Hah, thanks! I should have been a little bit clearer - not an ELI5-layperson, a layperson who has some vague handwavey idea of how video/film is made and once read a popular article about orange/teal contrast.
It's more stuff like 'what is it about this process that makes a dedicated colour specialist necessary?', 'what are the things things they're supposed to accomplish?', 'what are their technical and creative constraints/inputs/deliverables?', etc.
1. https://www.blackmagicdesign.com/products/davinciresolve/
2. https://www.filmlight.ltd.uk/products/baselight/overview_bl....