Mount it onto the Raspberry Pi and you have a stereo audio system. You only have to connect your loudspeakers. It provides up to 25W power. You only need a single 12-18V power supply to power the Raspberry Pi and the Amp+.
It powers the Raspberry Pi, there is no need to add an external 5V power supply to the Raspberry Pi. However if you want to use power-hungry USB components or other components (e.g. a display) you need to use a separate power supply (e.g. powered USB hub).