A ruleset is a set of instructions that Fantasy Grounds follows to run your game. It describes what makes up objects in your games: player character (PC), non-player character (NPC), item, spells, etc. and how they interact with each other. If you want to create a new object for your game, you can fill in the fields with information from your books and the program will understand this.
A ruleset does not automatically include any data. Data may or may not be available for a ruleset. Most of our commercial rulesets will also contain one or more data modules that represent the contents of rulebooks for that system. They may also include lists of monsters, items and spells.