In this recent DM Review article, Robert Blasum does a good job of defining and describing "business rules" and explaining why they matter in business intelligence. I'm linking to it because I remember this being a term that confused me for a long time when I first started working at SAS.
Loosely, Blausum says business rules "define how to interpret data." Here's why they matter:
Business rules form such a crucial part of performance management and BI because they give meaning to the numbers. Business rules allow one to interpret raw data, to come up with insightful reports and to use the information to propose actions. They are an absolute must for root-cause analysis and operational BI.