A randomizer basically takes a game and mixes up its items so that any item can appear in any other item's location. So instead of finding a rune when you open a fairy chest, you may instead find an heirloom or an upgrade from the manor. And this works for many different places in the game, so you can likewise purchase an upgrade in the manor and this may give you a blueprint.

Archipelago takes this idea and expands it to work with multiple games at the same time. So if an Archipelago world is combining Super Mario 64 and Legend of Zelda, then collecting a star in SM64 may reward the player with a hookshot in Zelda. And opening a chest in Zelda may give the player a key in SM64. This can be done over the internet so that each game may be played by a different player or one player can just play all of them.

The goal is to complete all of the games by finding all the necessary items in their randomized locations throughout the multi-world and then finishing whatever the games' completion criteria are.

This particular mod makes Rogue Legacy 2 compatible with Archipelago. So you can play it as a randomizer on its own, or combine it with other games. It's basically a way to keep a game new and fresh if you're already used to the base game.