Many elements of the game refer to time periods. The Core Set campaign is set in the time period 3. It has been defined that this campaign occurs right after the conclusion of the movie Star Wars: A New Hope. With that information and knowledge of the existing movies, this is what as been speculated as the reference for the time periods.
1: After Star Wars: Revenge of the Sith
2: During Star Wars : A New Hope
3: After the destruction of the Death Star, and before the battle of Hoth
4: During Star Wars : The Empire Srikes Back
5: After the "meeting" between Luke and Darth Vader, during the re-building of the Death Star.
6: During Star Wars : Return of the Jedi
7: After the destruction of the second Death Star, probably before Star Wars : The Force Awakens
In the rules reference guide, the time period are used as a filter to what allies, side mission, agenda cards can be used in reference to the campaign target Time period.