What's in the S.R.D.?

D&D is much older than magic. Even if there were a patent for the rules that wouldn't apply anymore, right? That is correct. So now what are rules in terms of copywriting versus what's in the S.R.D.? Is that a dungeon-y thing? We're on S.r.d. 5.1. 17 USC 102B says, in no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system or concept.