Aggregate Audio
Ive seen this talked about whats the point of aggregating audio just wondering incase i ever need to do it..always good to know an extra thing or two 
The point is to combine multiple sound cards into one. Alot of software can only use one soundcard at a time. This is simply a way of getting around that limitation.
As far as how to do it…you just build on in Audio MIDI Setup Utility. Set them all to have the same word clock source, the same sampling rate and bit depth, etc.. And it works better to aggregate multiple copies of the same card if possible, but it can work with different cards if you need it to.