Fallout Worlds offers players the ability to edit the settings of the game, removing build restrictions and upending game balance, then inviting friends along to play.
The online Fallout spin-off has long had private servers, but Worlds offers a way to tweak the configuration of your game. If you want to be able to build in previously restricted areas, you can do that. If you want to build enormous bases without electricity needs, or increase the difficulty and gift unlimited ammo, you can do that, too.
Fallout 76's private servers require a paid Fallout 1st subscription, but hosts can invite non-paying members to play with them. Bethesda themselves have also created several custom worlds of their own which are public and playable by all. In a blog post announcing the launch, Bethesda detailed the five public worlds available now, including "High Risk" which has no fast travel and always-on PvP and "Quantum World", which "lets you jump at maximum height with no fall damage."
I'm always torn about features like Fallout Worlds. On the one hand, it's a neat new feature for an online game. On the other hand, I grew up at a time when being able to type in console commands and edit server configs was the norm, not a new feature added years after launch. On the third hand, console commands and dedicated servers were inaccessible to most, and a clean in-game interface will almost certainly make this type of creative tweakery more broadly adopted.
In conclusion, I look forward to seeing what weird subcultures arise among the more popular custom servers, and also I am getting old.