Bricking occurs when some code is executed on the PSP that alters the Firmware to an unusable state (i.e. it formats the flash on the PSP). This means that the PSP will not function properly and therefore make the device unusable.
Bricking in the PSP may occur when using homebrew that alters the flash, such as using downgraders, custom firmware and other flash altering programs. Current downgrader/daters are 2.0 to 1.5, 1.5 to 1.0, 2.5 and 2.6 to 1.5, 2.71 to 1.5, 2.80 to 1.5, 3.03 to 1.5. It is wise that if one comes across one of these so called downgraders to look for some form of proof.