You will need a PSP running or ME Custom Firmware (CFW).

If you are using original hardware, press the Select button on the PSP home screen to open the VSH Menu. Change the UMD ISO MODE to Inferno . This driver provides the highest compatibility and fastest read speeds for repacked ISOs.

A "repack" is simply a modified version of the game files, often compressed for easier downloading. In this context, a "Manhunt 2 USA ISO uncensored repack" refers to a US region ISO file that has been pre-modified to remove the censorship.