ERML is a super easy-to-write DSL for representing conceptual data models via the Entity-Relationship modeling concepts.
You write ERML code (defined here) to represent your Entity-Relationship diagram, and the ERML Parser parses it into an AST, from there, an application can take the resulted AST and build all sorts of things with it, such as an ERD Visualizer.
For more detailed info on where the project is heading, check out the project's roadmap here.