In a press statement by Disney the plot synopsis of this Star Wars spin-off is as follows: "[Rogue One] tells the story of how a group of unlikely heroes unite to undertake a daring and seemingly impossible mission to steal the plans for the Death Star."
If you're not sure what on earth Rogue One is, let me explain. It's a Star Wars movie but it's not part of the new saga of films (beginning with Episode VII: The Force Awakens). It's a standalone film, set between Episodes III and Episode IV, focusing on the rebels who stole the plans for the first Death Star.