An InstantAtlas Data Catalog is a way to simplify the structuring, navigation and delivery of large volumes of statistics and indicators within ArcGIS Online. You can create a report from a Data Catalog using the New Report button in the Report Builder Editor top menu.
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.
You can use a Data Catalog as a data source for your report if you have a subscription for InstantAtlas Data Catalog or if you are a customer of the National Data Service.