It isn't since there is not much about what it is!

The first sentence of the main site is "D2: Declarative Diagramming - A modern language that turns text to diagrams", followed by multiple code snippets and the resulting diagrams.

I click the thingy and don't see that, I know D2 as the big language split for Dlang, I click around, I still can't tell what this is.

It's diagramming software? Like plantuml or mermaid? It says it right on the tin and with all the examples.