RPG stands for Role-Playing Game. It is a highly popular video game franchise that typically has the player take on the role of some character that they have the flexibility to customize. They must then embark on an adventure in a large open world to complete a larger overarching story.
Games like The Elder Scrolls, Fallout and Dragon Age are some of the most popular examples of RPGs. Other games like Dungeons & Dragons are often put unto the same genre.