The Report's Data Tab is where you'll find the micro-level details that make up your report. In a Contact Report, this would be a list of contacts who fit your filter criteria. In a Transaction Report, this is where you'd find a list of relevant transactions. You can search within this list, organize the list via columns or export this data to a CSV File to see it in greater detail.
Now that you've mastered the Report's Data Tab, let's learn about the Calculation Tab.