BBY/ABY is used to position items along the Star Wars Timeline. BBY refers to Before the Battle of Yavin. ABY refers to After the Battle of Yavin. The Battle of Yavin in Episode IV: A New Hope occured in 0 BBY/ABY.
When the Emperor takes control of the Jedi Temple and everything inside of it, Ferus Olin, a disgraced Jedi apprentice, and his street kid partner, Trever, must plunge into the depths of Coruscant to save the Temple.