Overview of Rollups
Rollups summarize data. For example, a rollup can calculate the sum of the potential revenue from the opportunities for an account hierarchy.
Rollup calculations appear on above the panels. This screenshot shows Opportunities and Service Requests rollups:

In Sales, the calculated value of a rollup appears in the object detail page. Here's a screenshot of an account in Sales, showing rollup attributes. This screenshot shows the Rollups tab on the left pane and the rollup attributes shown in a table:
You can view this video Rollups Explained on Oracle Cloud Customer Connect to learn how to create and enable a profile option that will expose rollup details in the UI and from where you can drill down to view an explanation of the rollup metrics.
For more information about viewing summaries of rollup metrics by aggregating key account information, including rolling the information up through an account hierarchy, see Sales Rollups Video Series on Oracle Cloud Customer Connect.