HiFiBerry DAC is a high performance sound card for your Raspberry Pi. Did you use external sound cards over USB in the past? There is no need to do this anymore. Just solder the P5 header on your Raspberry and connect the HiFiBerry DAC. The DAC is powered by the Raspberry Pi power supply, there is no need for an additional power supply.
You want to learn more about it: check out http://www.hifiberry.com