Priming a model means to use a spray can of primer on your models before you do any painting. You prime models so that when you paint that the paint will stick to the model better, giving you a better looking model.
Primed Fire Warriors
These are the primed version of the Fire Warriors.
Primed XV-8 BattleSuits
These are the bread and butter of the Tau Empire. XV-8's can fill any offesive role you can imagine. These mechs are one of the most versitile unit in the game.