By adding a sum field you can show the total of a specific field on linked cards.
For example: show the total number of estimated hours on a project card by displaying the sum of the estimated hours on the linked task cards.
This allows you to quickly get insights by only looking at top-level cards, without having to look at all underlying linked cards.
Start by changing the card type, next add a "sum" field.
Next you are asked to select the child card you want to consider. The last step is to select the field on the child card you want to sum.
Optionally you can set a filter on which child cards should be taken into account when calculating the total.
You can use the filter to create more complex reporting, for example: "Show the total number of hours remaining" by filtering on "Task" cards with status not yet completed.