It means that a battery is completely and totally discharged.  When Li-Ion cells in battery are deeply discharged, the copper ions will be released from anode.  But then the released copper ions may not be able to trace back to the original anode after battery is recharged.  Consequently, irregular spikes will occur and grow.  When the increment of spikes grows too much and over isolator, it causes short circuit and thus, battery is damaged.  The safety protection installed in ZAPAK Li-Ion battery prevents such hazard.