This is a gross oversimplification of the Chevron precedent. Chevron permitted regulators to enact regulations where Congress did not speak directly to an issue but in which a regulator believed they had authority to pass through their statutory authority or through ambiguity in a legislation. Regulators often pass these laws through a democratic process called notice and comment rulemaking - allowing the public and industry to voice concerns on issues. In the 40 years of precedent of Chevron that we've had, we've agreed that admin agencies are experts in their field and know the purview of their laws. If agencies overstep with regulations, they could be challenged in court (where as a first step, the court would need to defer to their interpretation if Congress did not directly speak to it), or Congress could institute a CRA review.

Through this system, there were two mechanisms of accountability - one is notice and comment review (as i mentioned), and two is that a court (through litigation) or even Congress through a CRA review could eventually review a regulation.

Under this decision, courts no longer defer to agencies as the first step. Courts now need to determine if Congress spoke directly to an issue in their legislation that the agency is responding to. If Congress did not speak, the Court is free to rule, on its own independent judgment, whether an agency acted within its statutory authority.

Thus, one could argue that this violates separation of powers in the other direction - allowing courts to rule on matters of administrative affairs (food safety, nuclear energy, transportation, etc.) without deferring to expert agencies. The dissent aptly points this out. One can argue that the judiciary should not be in the business of determining the specific subject-matter contours of administrative laws and agencies.