Aggregation badges are the small numbered icons that you see to the right of your column headers (field names) in table view, or list headers (labels) in Kanban view.
In Kanban view, by default these badges show the number of items with a particular label applied to them (AKA, the number of items in that list). They can also show the sum total of any number or formula field for all items with that label applied. To change the aggregation displayed for a Kanban list, just click the aggregation badge next to any list name, and select the option you want from the dropdown menu.
For example, in a collection you can set the aggregation badge to display the total material cost of all items under each label. This helps in forward planning and budgeting for your project. You can also choose which aggregation is displayed from the Kanban view options menu. Click on the name of your view then 'Edit'. Click 'Aggregation' then choose the field for which you'd like to see the sum total.
The aggregations in table view are slightly different. They show the aggregation numbers for all items in a collection or that match a certain filter, instead of just under a certain label. By default, the sum of number fields are shown, and the number of 'filled' items is shown for other fields.
For number fields, you can choose to display the sum, average, minimum, or maximum value of the items in that field. Click on the aggregation badge to choose which value is displayed.
For other fields you can currently only choose to display the total of filled or unfilled items in that field, i.e. the number of fields that have a value applied to them for that field.