Watch on the official Unity learn pages here -
http://unity3d.com/learn/tutorials/modules/beginner/5-pre-order-beta/send-and-receive-audio-effects

In this lesson we'll look at controlling our signal flow using Send and Receive effects. Send effects allow us to route a duplicate of an audio signal to another group or effect in our mixer enabling complex signal flows. We'll look at using sends to route signal to a reverb audio effect.