A Custom Firmware is a firmware for the PSP made by someone other than Sony. Dark_AleX is well known for his Custom Firmwares, including 2.71 SE, 3.02 OE and 3.03 OE. Custom Firmwares are used as a replacement to the Sony Firmware, usually to add extra features or to remove security features in Sony’s Firmwares. Installing a Custom Firmware can brick your PSP if instructions are not followed.
What is a Custom Firmware?