In the Star Wars universe, "ABY" stands for "After the Battle of Yavin," and "BBY" stands for "Before the Battle of Yavin." The Battle of Yavin refers to the climactic battle over the moon of Yavin IV, which is depicted in the first onscreen battle in all of Star Wars, the climax of "A New Hope," in which the Rebel Alliance destroyed the first Death Star. This pivotal moment represents year "zero" in Star Wars chronology. The BBY/ABY timeline is a standard for the measurement of dates in the Star Wars canon, and it has been the de facto standard since its introduction in 1994.
The BBY/ABY timeline is used to mark an epoch in the fictional Star Wars universe, and events in the Star Wars Legends continuity are typically dated in terms of years BBY or ABY. This dating system is used to tell fans when something takes place relative to "A New Hope," the first movie released in the Star Wars franchise.
In summary, "ABY" and "BBY" are used in the Star Wars universe as a dating system to mark events that occur before or after the Battle of Yavin, which is a significant event in the Star Wars timeline.
