From time to time, PHPFusion issue upgrades to your sites software.
These upgrades will contain a file called "upgrade.php" which is to be placed in the administration folder.
The database needs to be altered, if not for structural changes it need to update your PHPFusion´s version number.
It's very important that you always run the upgrade whenever one is released.
Visiting this page will tell you whether you need to update or not.