Archipelago: Multiworld Multi-Game Randomizer

This has really evolved over time. Originally, there were single-game randomizers, which became more and more sophisticated in what they could randomize. Early on, those randomizers added "logic", meaning that they ensure you can't have an impossible random seed where an item is stuck behind a point that needs that item to proceed, or a set of items that would mutually require each other.

At some point, people started combining games; most notably, "SMZ3" is a two-game randomizer for Super Metroid and Link to the Past (already two very popular games for randomizers), in which you're playing one SNES cartridge that has both games on it and you can step in specific doors to switch from a spot in one game to a spot in the other game.

Archipelago is an evolution of those multi-game randomizers, which combines the logic from each game to ensure that someone is always able to proceed.

And there are options you can choose to make things simpler; for instance, you can prevent certain key items from being very late in the game. But it's also fun to not do that, and end up (say) having to do half a game without an item you usually get in the first few minutes, working around the lack of that item. (For instance, playing substantial parts of a Zelda game without even a basic sword.)

Some randomizer modes also convert a more linear game into a more open-world game, by making it so you can go anywhere from nearly the beginning, though what you can do in those places will still be limited by the items you have.

The website has zero examples. Am I supposed to start a complex process and get friends on board based on something I've never seen?

TLDR context: a randomizer shuffles around game items/skills and even game areas such that the game is completed in a different order. Archipelago shuffles those things among different playthroughs. For example, I might pick up an item in my Pokemon playthrough that unlocks a gun in your Doom playthrough, and when you pick up a keycard it might unlock Ground Pound for a Mario player, and so on. It's like a giant collaborative puzzle game.

Here's an example with 114 playthroughs (includes some game spoilers in the latter half): https://youtu.be/YwUIfxF3ujo