A DAC is a digital to analog converter. To get the benefit of the DAC, you will want to use it's RCA (analog audio) outputs and run those to an input on your receiver. The idea here is that the DAC you purchase will have better sound quality than some other DAC you have access to. What other DACs do I mean? Well, there's probably a DAC built in to the NUC. If it has an analog audio output, then it has a DAC. There are also DACs inside your receiver. Any time you send input to your receiver via optical digital, coaxial digital, or HDMI (audio), you are going to use the DACs inside your receiver.
So you might not "need" a DAC at all. Also note that pretty much every consumer DAC is a two channel device: It plays Left and Right audio. If you intend to use this for multi-channel audio (5.1) or surround sound movies (5.1, 7.1, etc), then an external DAC is probably not what you want. At least not a normal 2 channel DAC.
I hope this gets you started in the right direction.
Brian.