If you'd forgotten about Bethesda's Creation Club, it's a marketplace of paid DLCs developed either internally by Bethesda or by approved external creators.
Here's the Skyrim Anniversary Edition content: If you buy Skyrim Anniversary Edition outright, what you'll be downloading is Skyrim Special Edition and a DLC package.
What's inclduded in the Skyrim Anniversary Edition? In practice, Skyrim Anniversary Edition is a DLC for Skyrim Special Edition, launching alongside a free game update fro Skyrim SE. It includes Skyrim SE and also bundles in all of the previously released premium Creation Club additions.
Okay, Skyrim AE is a bit like a Game Of The Year release for Skyrim Special Edition. Bethesda is calling the Anniversary Edition 'the most definitive version of Skyrim to date.' That's not the most concrete explanation, so it might help to think of Skyrim Anniversary as The Elder Scrolls V: Skyrim Special Edition - Definitive Edition.
