It mostly just contains graphical changes, and adds optional ray tracing which I wouldn’t suggest unless you have a very powerful computer.
It had some issues when it first came out, but it seems to work fine now from my experience. Don’t expect anything groundbreaking but it’s a nice update. Textures especially look better overall.
https://www.thewitcher.com/us/en/news/47105/next-gen-update-list-of-changes