When you import tables from a database, the existing database relationships between those tables is used to create the Data Model in Excel. The Data Model is transparent in Excel, but you can view and modify it directly using the Power Pivot add-in. The Data Model is discussed in more detail later in this tutorial. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.