The bottom line is that the Unreal Engine allows massively detailed images to be generated in real-time for either games, or like on The Mandalorian TV series where they use a dome with screens on all sides to generate the environment for the actors to act in. (https://techcrunch.com/2020/02/20/how-the-mandalorian-and-ilm-invisibly-reinvented-film-and-tv-production/)
The first version of the Unreal Engine was used to make a videogame in the late '90s called "Unreal". I played this game. By today's standards, it was very crude, but it was actually a step up from what we were used to at the time.