A ruleset is a bunch of settings for how scoring works - You can add new rulesets to the game that allow scoring to work differently.
For example, if you are an osu!mania player, you can use an osu!mania ruleset to show the accuracy you would get on osu!mania's engine.
Please note that while I've tried to make these simulations as accurate as possible, due to subtle engine differences the value might be off by up to 0.1%.
Which I recommend trying out SC, the standard ruleset, some existing simulations of other games are available through the import screen.
This lets you see how your scores match up if you've come from other games, or to have different kinds of sessions.
Maybe one day you want to go for super high accuracy scores, and another you want to train your speed. Sometimes one ruleset can be more useful than another, depending on what you're doing!
Rulesets are available for download ingame via the Imports menu.
Rulesets are a .json file that you can manually edit with any text editor.
There is currently no documentation for how the file works and users are currently not expected to be creating their own rulesets.
Want to create your own ruleset and need some documentation? Ask in the Discord.