So, okay, I am condensing like a semester of a class I took in 2017 into a very short explanation, but:

It would be really annoying for Congress to individually pass laws approving every new medicine or listing out every single poison you can’t have in tap water, so instead there are agencies created by Congress, via a law, to handle a specific thing. The agencies are created by Congress but overseen by the executive branch (so, the president), which is why we say things like “Reagan’s EPA” or “Biden’s DOJ” - even though Congress creates them, the president determines how they do the thing Congress wants them to do, by passing regulations like “you can’t dump cyanide in the local swimming pool” and “no, you can’t dump strychnine, either.”

However, sometimes people will oppose these regulations by saying that the agency is going beyond the task they were given by Congress. “The Clean Air Act only bans ‘pollutants,’ and nowhere in the law does it say that 'pollutants’ includes arsenic! You’re going beyond your mandate!” To which the experts at the EPA would be like, “We, the experts at the EPA, have decided arsenic is a pollutant.” On the flip side, the EPA could be like, “We, the experts at the EPA, have decided that arsenic isn’t a pollutant,” and people would oppose that regulation by being like, “But the Clean Air Act bans 'pollutants,’ and it’s insane to say that arsenic isn’t a pollutant!” So whose interpretation is correct, the government’s or the challengers’?

Chevron deference basically put heavy weight onto how the agency (i.e. the government) interpreted the law, with the assumption that the agency was in the right and needing pretty strong evidence that they were interpreting it wrong (like, blatantly doing the opposite of a clear part of the law or something). If there was any ambiguity in how the law was written, you’d defer to the agency’s interpretation, even if that interpretation was different depending on who was president at the time.

(Note: there are other ways of challenging regulations other than this one, like saying that they were promulgated in a way that is “arbitrary and capricious” – basically, not backed by any evidence/reasoning other than “we want it.” Lots of Trump-era regulations got smacked with this one, though I think they’d be better at it if Trump gets a second term, since they’ve now had practice.)

Chevron deference wasn’t all good – remember that the sword cuts both ways, including when dickholes are in power – but it was a very standard part of the law. Like, any opposition to a regulation would have some citation to be like “Chevron doesn’t apply here” and every defense would be like “Chevron absolutely applies here” and most of the time, the agency would win. Like, it was a fundamental aspect of law since the 80s.

The Supreme Court decision basically tosses that out, and says, “In a situation where the law is ambiguous, the court decides what it means.” That’s not completely insane – interpreting law is a thing judges normally do – but in a situation where the interpretation may hinge on something very complicated outside of the judge’s wheelhouse, you now cannot be like, “Your Honor, I promise you that the experts at NOAA know a lot about the weather and made this decision for a good reason.”

The main reason it’s a problem is that it allows judges to override agencies’ judgements about what you should do about a thing and what things you should be working on in the first place. However, I don’t think there’s really a way of enshrining that into law, outside of maybe adding something to the Administrative Procedure Act, and that would require a Congress that isn’t majority Republican.

I will say that kind of I expected this to happen, just because IIRC Gorsuch in particular hates Chevron deference. IMO it’s a classic case of “rules for me but not for thee” – Scalia and other conservatives used to rely on Chevron because they wanted their presidents to hold a ton of unchecked power (except for the EPA), but now that we’ve had Obama and Biden, now conservatives don’t like Chevron because it gives the presidents they don’t like unchecked power.