A ruleset is a bunch of settings for how scoring/accuracy works - You can add new rulesets to the game that allow scoring to work differently.
The default (and recommended) ruleset is SC J4, this is the scoring system that all online leaderboards use, but 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.
Rulesets can be swapped at any time and you will see all your scores converted to their accuracy in that ruleset.
There are pre-made rulesets for osu!mania, Quaver and Etterna accuracy systems available through the ingame options menus.
Rulesets can be edited ingame via the same menu used to view and select them - You can customise them to your hearts content.
You can rename or change the colors of judgements, or totally change the timing windows, releases and note locking mechanics if you want.
