It's originally about Chevron the company lol. The original Supreme Court case that set the modern precedent was Chevron (the company) vs. Natural Resources Defense Council. That case allowed the EPA to do things like determine safe levels for things like lead in water, particulate matter in air, etc. without explicitly having a whole ass court argument over each and every number that they wanted to set.
Overturning the original Chevron case will literally dismantle the entire regulatory process in the United States.