What is a wet coat?

A mist coat is when you're building up a foundation of the paint color, the paint should be pretty much dry as it hits the model by letting out less paint, at a lower pressure, and you thin the paint more than normal.

You do a couple layers of that, then the wet coat is when you let more paint out of the airbrush so the paint is no longer going on and drying quickly, but it's actually looking wet. If you have a good leveling thinner or leveling gloss paint that'll give it the good solid shine look. It kind of helps avoid the orange peel look of paints that are sprayed or dry unevenly. 

Mist coat is like a light misty rain on a windshield.....enough to see, but not really enough to turn on the wipers.  Probably less than 50% coverage in a thin light coat.

As in the previous answer, wet coat is an even shiny coat, but not so heavy that you get runs.

Practice on scrap plastic or cheap for sale/garage signs.  Dollar store items.  Even plastic fast food containers.