Here is my understanding of the issue. National Marine Fisheries set a regulation that fishing boats need to pay the salary and expenses of the people that are stationed on their vessels by NMFS to monitor the catch and bycatch on a commercial vessel. Some fishermen sued saying it was an undo burden or something, but the case has kind a blown up into looking at a 40 year precedent as to whether or agencies get deference for setting regulations and rules to implement the laws that congress passes. Basically, agencies get a directive from congress and then make a plan for implementing the law by a process called rule making because Congress doesn't often spell out exactly how to implement a law. My experience with this is, for example, that the endangered species act says that the FWS has to assess whether a species needs protection and then FWS has to provide that protection if warranted. But, congress never specified what they mean by "needs protection" so the FWS comes up with some rules and definitions for that concept. One problem with this is that with each new administration, rules and regulations can be revised which causes headaches for businesses trying to comply. Some case involving Chevron in the 80's solidified that concept that agencies interpret and implement the laws. But, in this case with the fishermen, conservatives are trying to argue that congress never mandated observers on the boats let alone that the fishermen had to cover their pay and that the NMFS has no authority to assert this rule. Rational people are left to argue that is the way it has always worked and that Congress really should be getting involved in the nitty gritty of implementation. In an extreme scenario, if they overturn the precedent regulatory agencies won't be able to do anything without Congress specifically say so. It would bog down even the most functional efficient legislative body.