A custom summary formula is a formula that calculates values from summary fields in reports such as sum, average, min, max, etc. Custom summary formulas can use functions such as PREVGROUPVAL to compare values from different groups in reports. For example, to calculate the year-over-year change in dollar amount of opportunities, an administrator can create a custom summary formula that uses PREVGROUPVAL to compare the sum of amount from this year's group with the sum of amount from last year's group, and divide it by last year's sum of amount. References:
https://help.salesforce.com/s/articleView?id=sf.reports_builder_fields_summary_formulas.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.reports_formulas_prevgroupval.htm&type=5