Mix parameter will be probably tweaked a little bit so it's more intuitive to use. Basically it randomly mixes between input colors/maps:
Imagine you have a forest and you want your trees to be randomly slightly different shades of green.
Previously you would have to use say at least 5 texture maps or colors with slightly different hue between for example yellowish green and rich green.
With mix, you can simply use just two slots, and put very green leaf texture in one, very yellowish or dry leaf texture into other, and raise mix amount. Now all the trees in your forest will have random colors somewhere between those green and those yellow leaves.