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 Rebellion's fight to liberate Mon Cala continues as Han, Luke and Leia undertake a perilous and nigh-impossible rescue