Complex sentence: Structural metadata is about the design and specification of data structures and is more properly called "data about the containers of data"; descriptive metadata, on the other hand, is about individual instances of application data, the data content.
Split into simpler sentences: Structural metadata is about the design and specification of data structures and is more properly called "data about the containers of data". Descriptive metadata, on the other hand, is about individual instances of application data, the data content.