Unity Catalog in Databricks is a unified governance layer for data and AI assets, centered on a metastore and a 3‑level namespace (catalog → schema → object). It matters because it standardizes access control, auditing, lineage, and sharing across workspaces and compute types. A useful mental model is “secure by default”: to query an object you typically need both container privileges (for the catalog and schema) and an object privilege (like SELECT), and governance objects (like external locations) gate storage access.