The List View shows Opportunities in a grouped table of records. The Group By menu allow users to group and subtotal deals in different ways in. Several options come out-of-the-box, and admins can add additional options.
Groupings Side Panel
Click "more groupings" in the Group By menu to open the Groupings Side Panel. In the side panel, you can choose to group on any of the preset options, any groupable field on the Opportunity Object, or any groupable field on the the Related Objects (i.e. Account > Type or Owner > Country).
Using the Groupings Side Panel, you can also add multiple groupings to created a nested list view with subtotals for each level.