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.
It has all built up to this! Vader's trials against Cylo's creations! His machinations against the Emperor! His covert missions with Doctor Aphra and her murderous droids! All comes to fruition in an ending you can't miss!