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.
"AWAKENINGS," PART FIVE! BLACK SQUADRON had its own mission during the events of THE LAST JEDI, a desperate trek across the galaxy to find aid for THE RESISTANCE, all without the help of their fearless leader, POE DAMERON. This is the beginning of the end.