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.
After the events of Order 66, survivors of the Jedi Purge must navigate the galaxy undetected. It's been a year since Ahsoka Tano watched the Galactic Republic fall, and she's leaving yet another planet to dodge Imperial suspicion and start over. Again. Something strange is happening, however, and she can't help but let her curiosity get her into serious trouble.
E.K. Johnston's young adult novel builds an important bridge between the Ahsoka we knew from The Clone Wars and the Fulcrum we meet in Star Wars Rebels. It's in this story that our hero must come to terms with her past and discover the role she is meant to play in her darkened galaxy's future.