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.
As the threat of the Jedi continues to menace the fledgling Empire, Vader finds himself on the trail of a Jedi his master has commanded him…to keep alive?