Each record in your data source will repeat once within the catalog merge area. The catalog merge area will repeat one or more times per page in the merged pages until all of the selected records from your data source are displayed.
If you are not sure what type of information you need to include in your data source, it might help to "think backwards."
Imagine your final merged publication pages. What kinds of items do you want to display in your publication? What specific types of information do you want to provide about each of these items?
Your data source should include a record (row) for each item that you want to list in your catalog. It should also include separate data fields (columns) for each unit of information that you want to include per record.
For example, imagine that you are creating a catalog for the Alpine Ski House. The catalog needs to list all of the activities that guests can enjoy while staying at the lodge: heli-skiing, dog sledding, ice skating, etc. Each of these activities should be a unique record in your data source. Your data source should include a separate data field for each piece of information about the activity that you want to display (activity name, activity description, photo, regular price, special price).