What does it mean to say a condition is natural?
It means that the condition was necessarily caused by some prior condition according to a rule. That the natural condition was determined by past facts and laws.
That means that a non-natural (supernatural) condition is one that is not so determined. A supernatural condition is one that is random, and inexplicable. There's no explanation for such a condition because one cannot (in principle) point to any facts of past conditions that predict it.
Naturalism rejects the idea that there are supernatural explanations. That's because it is nonsensical to say that an event is both explained and inexplicable.
As I've said before, the naturalist need not make any strong negative claims about the non-existence of the supernatural. There's no need to say that nothing supernatural ever happens. Supernatural things might happen every day, and those things would appear to us as unexplained events.
Well, I've come to believe that one can say that there may well be at least one supernatural thing: the sum of all laws of the universe. There cannot be any explanation for the universe itself, because that would require some set of external or prior circumstances that determined its state. Yet any such determinant would require new laws, in which case, we're no longer explaining the universe, but only a part of it.
I don't see that my conclusion makes any significant difference to the substance of these debates, but it does help to answer those who recognize that there are some questions that cannot be answered by natural laws. With them I will agree, but that doesn't really help their case. It simply means that there are some questions that cannot be answered at all.