Wet palette is basically a palette that is moist and when you have paints on a moist surface they do not dry as fast as usual.
They are cheap and easy to make.
http://www.youtube.com/watch?feature=player_embedded&v=-tIciEHhwDE#! I recommend that you get a container that has a low profile and a lid.
Also as a friendly advise you could just these simple questions. You would get your answer much faster.
ie. "what is a wet palette" and "how to make a wet palette".