...

You can add multiple Rollup Types, such as Average, Concat, Count, Count Distinct, List, Maximum, Minimum, and Sum, within a single rollup by using the Add Rollup Action option.

What is source field?

• In this section, users Users can select the source field from which to perform rollup calculations or derive data.

...

What is destination field?

• In this section, users Users can select the field where the result of the rollup calculation will be displayed.

...