Complex sentence: During the events of "Rogue One", Jyn is a member of the Rogue One team, assisting in the search to destroy the plans for the Death Star, an armored battle station which has the ability to destroy entire planets.
Split into simpler sentences: During the events of "Rogue One", Jyn Erso was on the Rogue One team, assisting in the search to destroy the plans for the Death Star. The Death Star was an armored battle station which had the ability to destroy entire planets.