What does zero Iq means in reverse voltage protection ICs
I_q means "quiescent current" in this context and it's the current drawn while operational but not under load, see page 5 of the datasheet you linked. In general the lower I_q, the better, because the efficiency will be higher. 

Check some background info on I_q here: http://www.ti.com/lit/an/slyt412/slyt412.pdf