In a relational database, foreign keys are normally used to associate records stored in different tables, but wouldn’t it be nice to define relationships dynamically without having to add extra columns or tables? And while we’re at it, how about having sparse relationships by associating a record directly with any other record like “post X was last edited by user #123” or “post X was flagged for review by user #456” (who happens to be a moderator)?
AFAIK, no NoSQL database fully supports SQL, and only some offer support for transactions and joins. The idea here is to augment a relational database by adding capabilities for dynamic relationships.