it was an area in china that was occupied by japan during world war 2, while they occupied it, they raped their women and made them into "comfort women" for their soldiers, looted, killed, and performed pratically every single known attrocity to man to it's inhabitants
the worst part is that when the war ended, pratically all of japan's records of the event were destroyed in a desperate attempt to cover it up.
the rape of nanking has been called the forgotten holocaust, japan's most recent history textbooks either don't mention the event or call it the "incident in nanking" in an attempt to downplay the tragedy
to this day no formal apology from the government has ever been given to the victims, any anti-japanese sentiments that most asians have is most likely due to this incident