You can create a report from a data catalog using the New Report button on the Report Builder home page.
An InstantAtlas Data Catalog is a specific way of simplifying the structuring, navigation and delivery of large volumes of statistics and indicators within ArcGIS Online. It is the way in which Esri UK delivers data for the National Data Service as a managed service.
A data catalog consists of a master table that indexes indicator data stored across a series of feature services. The master table allows the indicators to be organised into a theme hierarchy, which then makes it much easier to find the indicator(s) that you are looking for in Report Builder.
Another advantage is that it makes it easy to set your widgets to pick up the latest X dates from the indicator time series, so that the widget will automatically update when new dates are added to the source data.