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 Empire's power grows, rebels face a choice: Give in...or fight. Some form resistance on battleground planets. Some go undercover, trying to fight the Empire from within. And others watch and wait.