Rulesets describe the conditions under which a resource should be subject to an approval process.
A simple MODX website would typically require only one ruleset that ensures all resources are approved by a specific user group before it is published. More complicated organisations may setup multiple rulesets to describe alternative approval processes for different contexts, templates or class keys.
For example, on an ecommerce website it may be nessesery for all resources in the "shop" context to follow a ruleset that requires a Product Manager to approve a product before it is published. Whilst within the main "web" context the Content Manager should approve revisions. This scenario would be setup using two rulesets.
Within each ruleset is the abilty to design each stage of the approval process, using the drag-and-drop designer.