The Hasura Data Dictionary is an open-source project designed to help developers understand their data graph, including database relationships and GraphQL schemas. It also assists in creating complex GraphQL queries. The Data Dictionary provides a visual entity-relationship view of table relationships and a GraphiQL tab for experimenting with queries. Currently supporting Postgres databases, future updates will include MySQL and SQL Server support. Developers can contribute to the project and use it as a starting point for their specific needs.