VTT is a virtual tabletop. It's useful to be able to load the game's rules into the software so it can keep track of stats, equipment, etc.

For more context:

Hasbro/Wizards of the Coast (the company that has owned D&D for the last 20 years) released the rules text and some basic creative content like classes, some monsters, etc. for D&D 3.5 under a custom open source inspired license called the OGL. It's sort of a weird blend of GPL-esque virality clauses but the actual restrictions it places are much more akin to a permissive license like Apache. This led to an explosion of third party content.

When D&D 4e came around, Wizards felt they didn't want to compete with that so they released D&D 4e under a new license called the GSL which was much more restrictive to allow them to shut down competitors, which caused third parties to shun 4e and continue publishing 3.5e content.

Partly because of the continued existence of the 3.5e ecosystem (along with some controversial video game inspired gameplay changes), 4e largely failed and alienated the fanbase. So they released 5e with a number of peace offerings to the community, one of which was that 5e would use the OGL again.

Now Wizards are on the cusp of releasing OneD&D (6e). Part of their strategy for 6e is their own virtual tabletop (VTT) product. A virtual tabletop is effectively a software product which gives players a chatroom + shared map and drawing tools so they can effectively play tabletop games which are designed around more props online. Wizards plan for their own VTT includes a very video game inspired monetisation model where they would sell skins, spells, subclasses etc.

The problem they have is this: they are a very late comer to the VTT market, since they haven't launched their product yet, while their competitors (especially roll20) have been going for near a decade now. And these competitors were perfectly in their right under the OGL 1.0a license to use enough content to enable their players to play D&D on it. Wizards did not feel having more content and a higher fidelity product was going to be sufficient to drive players to their microtransaction filled product, so they did not want to release 6e under OGL.

But they remembered the 3.5e/4e problem. They didn't want to release their new, more locked down 6e and have everyone just keep playing 5e and 5e derived content under the OGL. So they decided they would try exploit some of the wording of OGL 1.0a to de-authorize it and replace it with a new OGL 1.1 which was a very draconian license given Wizards royalty-free unlimited licenses to other 1.1 content, yet imposing royalties, attribution, registration and field of use restrictions on third parties. That way nobody could keep making 5e VTT plugins, 5e addons, etc. and everyone would have to go to 6e and the competitor VTTs would die from not being able to support the most popular game.

This leaked when they asked third parties to sign it, to much bad press, so Wizards announced OGL 1.2. It did roll back many of the more egregious restrictions from the 1.1 version, but it still kept veto power with wizards (wrapped up in an anti-hateful content clause, but one that was so broad and required waiving a right to contest to the extent wizards could define competing too closely with them as hateful behaviour), and it still put limits on what VTTs could do and allowed them to change the rights of VTTs at any moment.

Players still felt this was not enough and did largely vote with their wallet by cancelling subscriptions to D&D beyond, D&D's online content distribution and character builder service that was to be the baseline of the new planned VTT, to such a level that Wizards have now had to capitulate and roll back their planned changes to existing and older editions at least.

This is an excellent explanation, thank you!