MongoDB stores system information in collections that use the <database>.system.* namespace, which MongoDB reserves for internal use. Do not create collections that begin with system.
MongoDB also stores some additional instance-local metadata in the local database, specifically for replication purposes.
System collections include these collections stored directly in the database:
The <database>.system.namespaces collection contains information about all of the database’s collections. Additional namespace metadata exists in the database.ns files and is opaque to database users.
The <database>.system.profile collection stores database profiling information. For information on profiling, see Database Profiling.