Let’s say you have a Glide Table of sales data and you would like to determine sums by month.
You would then create a separated Glide Table called Monthly Sales Report, create 12 rows for 12 months (January, February, … , December), create a relation or query column to point each month to the associated rows in the Sales table, and finally you would create a rollup column and configure it to sum.
This thread on the query column could come in handy.