All you need to know about Rulesets in Worldwide Combos.
Rulesets determine the rules of the rounds that you are going to play, either in Quickplay or in Custom rooms with basic settings. They specify some parameters that will influence, for instance, how to send lines, how long the rounds will last, etc...
As of now there are eight official rulesets, which are enabled by default in Quickplay:
Custom rulesets may be created by players owning a Premium account, once a custom ruleset is made all players may join it in Quickplay through the filters menu.