If you insert a PS2 memory card with free mcboot on it the PS2 will load a different menu than it normally does. This menu can be configured to autoboot different software, but basically it lets you run homebrew. There's emulators, burned game loaders, other stuff. When installing free mcboot there's 2 install types. One is specific for your console (so if you did that install and then tried to use your free mcboot memory card on another machine it wouldn't work) and one is specific for your region (card would work on every machine in your region, but none out of region.)