Rogue One is a Star Wars Story because it expands the Star Wars universe by focusing on a specific, standalone mission that ties into the broader saga.
Rogue One: A Star Wars Story's plot revolves around a ragtag group of individuals. These include Jyn Erso, whose father was forced to work on the Death Star. The rebels realize they need to get the plans to find a weakness in the Death Star. They embark on a perilous journey, infiltrating various Imperial strongholds. There are epic battles and tense moments. In the end, they succeed in getting the plans, but at a great cost, as most of them sacrifice themselves, setting the stage for the events in the original Star Wars movie.
Rogue One is closely linked to the Star Wars story as it provides a crucial backstory and sets the stage for the events of the original trilogy. It shows how the Rebel Alliance obtained the plans for the Death Star.