Well, the subtitle 'Solo: A Star Wars Story' is really important. It clearly indicates that this movie is a part of the Star Wars universe. It tells the story of Han Solo, a beloved character. The movie explores his early days, how he became the scoundrel we know from the main Star Wars movies. For example, it might show how he got his ship, the Millennium Falcon, and how he met his friends like Chewbacca. It gives a deeper understanding of the character's origin.
The subtitle gives a hint that the movie focuses on the character of Han Solo. It's a way to attract fans of the Star Wars franchise who are interested in Solo's backstory.