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 movie The Empire Strikes Back shows us only a small portion of Luke Skywalker's Jedi training under the tutelage of Master Yoda. In this story, witness a never-before-seen part of that training as Luke is given the task of finding -- and retrieving -- an object guarded by the deadliest creatures on the swamp world of Dagobah: the monstrous dragonsnakes! This is the most harrowing, dangerous, and muddiest mission that Luke has ever faced!