In SQL Server the data dictionary is a set of database tables used to store information about a database’s definition.  The dictionary contains information about database objects such as tables, indexes, columns, datatypes, and views.

You can use this online version to learn about the AdventureWorks table and columns:

Adventure Works Data Dictionary Report (online)

You can also download a PDF version.

Either is handy if you're not setting up your own SQL Server learning lab and need to know more about the table structures to answer the course questions.