Mixing is when your radio does two, three or four different things with one input. Most scale aerobatics (IMAC, IMAA) have some sort of a mix. Lets say you start an incline and you see that the plane starts moving a little left on you while you are climbing. Well if your radio has mixing you can set your rudder to slightly move with the elevator so one movement(down elevator which causes lift) will also move your rudder so there is no left or right movement of the aircraft. Like I said its much more prevalent in aerobatic type flying where you need precision. Dual rates is allowing your plane to handle two different ways with a switch of a button. You could program your sticks where your plane turns slowly and then program the high rate to make the plane turn quicker. Its a touchy subject with dual rates but thats basically it.