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.
alt Hacker News
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.