Data about a piece of data. Examples include

The FileAllocationTable of a FAT FileSystem

describes the on-disk distribution of the data that makes up each file.

The journal in a JournaledFileSystem?

describes the actions to be taken upon the MetaData of a file in the FileSystem it resides on.

The control file in a Deb Package for DebianLinux

specifies the content of the package and its relation to other packages.

Bibliographic records (such as per the MARC and Dublin Core standards)

contain information like the Title, Author, Date, etc of documents in a database in order to ease look-up.

The list is endless. Even this page you're reading is MetaData, it is a description of MetaData.