From the MDN article, it sounds like it's basically the same as plain old CSS variables but scoped to specific elements and/or children with some additional constraints so that it has some awareness of CSS units like angles, percentages, etc...
I probably won't be using it though. Such slight benefits don't justify added complexity and compatibility tradeoffs. I try to avoid using unnecessary new fancy syntax. I kind of stopped caring about new additions to CSS after CSS variables.
I'm sure it's very difficult and stressful to be a surgeon, but once you receive that big paycheck at the end of the week/month while sitting comfortably in your expensive home with a view, scrolling through your stock portfolio on your phone, thinking about what shares/companies you will buy next and where your next holiday will be, it can't be that hard to recover!