It means a judge can, if they so choose, decide if an ATF rule is lawful or not instead of the ATF itself always having the final say. Speaking of the ATF, it took a Supreme Court ruling to get ammonium perchlorate off the explosives list so model rocketry could continue as a hobby. With this ruling, a case like that wouldn’t have to go all the way to the Supreme Court. This is a good thing.

They won't be able to stretch the law as much as they have at times. As the court's decision in Raimondo and the concurrences note, the court had already stopped using Chevron since 2016. Notably the court ignored Chevron last week when they decided that bump stocks are not machine guns -- if you read that decision it was all about the interpretation of "more than one shot with a single action of the trigger", which is precisely what the court would have deferred to the ATF on under Chevron and which it did not now.

The scaffold of federal gun control (NFA etc.) remains untouched by Raimondo. Only ATF rulings regarding various technologies developed in the past 40 years will be affected, and probably not that very many. I doubt more than a very small handful of ATF rulings will be affected.