ABAP 4 data dictionary describes the logical structures of
the objects used in application development and shows how
they are mapped to the underlying relational database in
tables/views.

Data definitons (metadata) are created and managed 
in the ABAP dictionary. The ABAP Dictionary permits a 
central Description of all the data used in the system 
without redundancies. New or modified information is 
automatically provided for all the system components. This 
ensures data integrity , data consistency and data security.

    you can create the corresponding objects(tables or 
views)in the underlying relational database using these data
definitions.The ABAP dictionary therefore describes the 
logical structure of the objects used in application 
development and shows how they are mapped to the 
underlaying relation database in tables or views