It's something like global variables for every client (can be used for health, armor, ammo,...). But I'm currently thinking about removing this feature because you can easily do the same with a global array.
My plans for ANet are to make things more simple and compact again. I don't think that playervars were ever used by users. That new mantra is also the reason why I introduced enet_sendto() (1 function for all so to say).
If nobody is using the playervar feature, I will delete it. So please tell me if any of you want this feature to stay