The basic idea is that some states require a person to retreat from threats, even in their own homes in some cases. Self-defense, under these laws, can only be claimed if escape is impossible. "Stand Your Ground" laws make it explicit that retreat is not legally required. A law-abiding citizen can simply stand their ground and if someone insists on attacking them then force may be used for self-defense.
Seriously, how do we know that the knocks to Zimmerman’s noggin by Martin didn’t disorient him? If Zimmerman had not been ambushed and thumped my guess is he would have reacted differently.
Equally important, “stand your ground” doesn’t even really seem to apply in this case because, accoding to all available evidence, Martin had Zimmerman on the ground, beating the living Hell out of him, and Zimmerman’s only choice was to discharge his weapoon or be killed or severely injured. IOW this is a standard self defense case, not a stand your ground one.