Metadb is a new reporting database architecture in beta release that will eventually replace the Library Data Platform (LDP) software.
Resources
(draft) Metadb documentation is available here: https://d1f3dtrg62pav.cloudfront.net/doc/
This is where the Metadb-based query development is being done for FOLIO: https://github.com/folio-org/folio-analytics/tree/main/sql_metadb