[genderwhy]

WotC licenses some of their content using the Open Gaming License. It ostensibly covers both the rules* of D&D as well as key elements of the setting -- particular monsters, characters, place names, spells, etc.

That license has allowed products and content creators to build on top of a shared platform -- using and reprinting portions of D&D's content to build their own worlds, stories, systems, etc. Note: not everything D&D publishes is covered by the OGL, just a set of core items they call the SRD -- Systems Reference Document.

WotC/Hasbro leaked that they were working on OGL 1.1 which had a bunch of ambiguous (and many argued harmful) language that required creators to do things like license their content back to WotC, pay fees to license content, control what and what was not appropriate to build on top of OGL, etc.

The OGL 1.1 was met with huge community backlash, and wotc has been fumbling for some time to figure out the next steps. It looks like they are taking those steps now.

* Aside, it's not clear that the rules of D&D are even something that can be licensed in this way, as game mechanics are not protected the same way as copyrightable characters are.