Just like the name suggests. They are a way to coordinate the control surfaces on your aircraft. For example: say that when you add throttle to you plane is wants to climb but you want it to stay level. You can mix the throttle channel and your elevator channel to correct that. Basically you are saying that for every "x" percent of throttle added the elevator will push the plane back down to compensate.
Check out this post on RCgroups. Very good explanation: http://www.rcgroups.com/forums/showpost.php?p=5134979&postcount=13