What does it mean "As-of Date"?

Hi 

I would like to know the functionallity of parameter "As-of Date" of report Create Customer Statement , because I tried to run it for September month, but the report is not generated.

However, when I complete this paramenter "As-of Date" with default date (today), I get the report.

What does it mean "As-of Date"?

Regards.

Romina