A custom summary formula is a formula that calculates values from summary fields in reports such as sums, averages, counts, etc. Custom summary formulas can use functions such as PREVGROUPVAL which returns the value of a field from the previous row at the same grouping level. By using a custom summary formula with the PREVGROUPVAL function, an administrator can create a report to calculate the year-over-year change in the dollar amount of a company's opportunities by subtracting the amount from the previous year from the amount from the current year and dividing by the amount from the previous year. References:
https://help.salesforce.com/s/articleView?id=sf.reports_builder_summary_formulas.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.reports_builder_functions.htm&type=5