In the documentation at https://confluence.atlassian.com/jirasoftwareserver/reporting-939938675.html for "Created vs Resolved Issues Report", there is a bullet point in the expanded area that says:

"Viewing the chart — Areas in red show periods where more issues were created than resolved. Areas in green show periods where more were resolved than created."

My suggestion is to add another bullet point below that with the text:

"The optional Trend of Unresolved chart shows the cumulative change over time of Created less Resolved numbers. If the blue line is horizontal then you are creating and resolving issues at an equal rate. If the line is going up then you are creating more issues than you are resolving. If the line is going down then you are resolving more issues than you are creating"