Much like with The Force Awakens, there will be some official canon tie-in novels and comics giving some back story to the characters in Rogue One. One of the first will be James Luceno’s Rogue One: Catalyst that focuses on Director Krennic and Jyn Erso’s parents in a story set during the time of The Clone Wars.
Here’s the official synopsis:
“War is tearing the galaxy apart. For years the Republic and the Separatists have battled across the stars, each building more and more deadly technology in an attempt to win the war. As a member of Chancellor Palpatineâ€™s top secret Death Star project, Orson Krennic is determined to develop a superweapon before their enemies can. And an old friend of Krennicâ€™s, the brilliant scientist Galen Erso, could be the key.
Galenâ€™s energy-focused research has captured the attention of both Krennic and his foes, making the scientist a crucial pawn in the galactic conflict. But after Krennic rescues Galen, his wife, Lyra, and their young daughter, Jyn, from Separatist kidnappers, the Erso family is deeply in Krennicâ€™s debt. Krennic then offers Galen an extraordinary opportunity: to continue his scientific studies with every resource put utterly at his disposal. While Galen and Lyra believe that his energy research will be used purely in altruistic ways, Krennic has other plans that will finally make the Death Star a reality. Trapped in their benefactorâ€™s tightening grasp, the Ersos must untangle Krennicâ€™s web of deception to save themselves and the galaxy itself.”
The Celebration teaser for Rogue One showed a scene of young Jyn, so that confirms that we’ll also see this Clone Wars-era in the movie itself via flashbacks.