MIDI controller is any piece of equipment that generates and transmits MIDI data to MIDI-enabled devices. In short, if you have buttons on your MIDI controller, you can program those buttons to any sound you want through musical software (ex.: Ableton, Garage Band, etc.). You can also program potentiometers to control effects, volumes, etc..
This instructable will show you how to create your own MIDI Controller using Arduino. With a MIDI controller, you are rarely limited with what you can do. There is endless possibilities and endless fun.