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.
The Jedi Temple is attacked, an Emperor is betrayed, and the Sith are born anew! A lot can happen in a hundred years, but that's just the beginning of the story! This is a perfect jumping-on point for any reader - an epic beginning to an untold chapter of the greatest adventure in the universe!