A report has a measure that for calculation uses the result of a previously calculated measure. Using cache would store the first time calculation result in memory and for the next measure would be used to optimize the performance of the report.
A default measure in Jira issue cube Open issues uses cache function for performance improvements as the query should go through all time members and repeat the mathematical expression to each of them.
See sample report Created vs resolved issues over time in our Demo account. Measure Open issues contains the formula above.