The Open Game License (OGL), released by Wizards of the Coast in 2000, lets anyone freely use and redistribute part of a game’s rules ("Open Game Content"). The document organizing that usable rule set is the System Reference Document (SRD).
Thanks to these documents, rules and concepts such as the zombie, skeleton, and saving throw can be cited and restated legally. But proprietary names and designs excluded from the OGL/SRD as Product Identity — like the beholder and mind flayer — remain protected, so this hub treats the line between name and concept with care.