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.
Darth Vader has returned to Coruscant after the destruction of the first Death Star. There, he discovers his next mission. In the days before Grand Moff Tarkin's death, Tarkin recorded a top secret holotape... complete with information that would enable the Empire to dominate hyperspace. Your mission: to retrieve the missing holotape and bring glory to the Emperor.