It is a security to a report page at the User level.In PLS, 
user can able to see only those pages to which he has given 
a privelege to access.

A report developer can use page-level security to determine 
user access to specific pages of an Actuate basic report. 
Using page level security, a report developer can create a 
report having security rules that control which pages a 
user can view. To view a report that uses page-level 
security, a user must have secure read privilege on the 
report.
Page level security options :- Read, Write, Secure Read, 
Visible, delete.

Thanks-