Recommended fields enable you to pare down the list of fields shown in the report editor. Particularly for data source that might have dozens of folders and hundreds of fields, this can help make report development easier by limiting the fields to the handful you normally use.
By default, recommended fields are pulled from the prebuilt templates in Easy Insight if any recommended fields exist.
You can customize these recommended fields by going to data source configuration -> Recommended Fields. The toggle at the top allows you to choose whether or not you're customizing recommended fields:
You can choose to add all fields from a child data source or you can choose to add a particular set of fields from a child data source:
After saving, you'll see that the Recommended Fields toggle is available when you add a field to a report or create a filter on a field. When selected, you'll only see the subset in the recommended fields: