The Mbox is an audio interface. It is used to replace the soundcard in your computer, as these are poor quality and not suited to recording. It is a way of getting high-quality sound in and out of your computer.
The most popular way to record guitar is to mic the amp. The mic signal is fed through the mbox into the computer.
You can also record the guitar direct, then use 'amp modelling / simulation' plugins on the computer to add distortion or effects.