Metadata in DBMS refers to data about data, which describes the context and information about data, the way that data is stored, and the various relations among data. It helps in understanding, finding, and organizing data. In a relational database management system (RDBMS), metadata is stored in a structure called a data dictionary or system catalog, which holds information about tables, columns, data types, table relationships, constraints, and more. Metadata is used by developers to develop programs and queries to manage and manipulate data. There are four types of metadata in DBMS: technical metadata, business metadata, operational metadata, and external metadata. Technical metadata gives data about the structure of the database, while business metadata provides information about the business context of the data. Operational metadata provides information about the execution of the system, and external metadata provides information about the data that is not stored in the database. In summary, metadata is an essential part of a database system that provides information about the data and helps in organizing and understanding it.