AFAIK it’s a PvE game that utilizes the same mechanics as the battle royale game, but otherwise is mostly separate. I think if you purchase it you get a bunch of XP for Fortnite BR by playing it each season though

https://www.fortnite.com/save-the-world

I’m sure you already heard of it, but obligatory Voyager shout-out since it’s pretty damn close

https://github.com/aeharding/voyager