In simple words,
BRM provides web based rule management system. Rules/tables/scripts are built/maintained upon web service model. You can easily access/modify/test your business rules and values in internet browser. (Encapsulation)
Any business person can easily understand and handle business rule/values of any organization's processes without having any technical expertise in a simple web based UI. He don't have to open/login into tools to control your business processes. (Abstraction)
It provides loosely coupling of your business logic with integrated process. So basically you can design/use a standard integrated solution with different behaviour based on your business rule engine (Polymorphism).
Business Rules Management - SAP Library
Creating and testing flow rule-set using Rules Composer in NWDS and executing it from WSNavigator
Thanks,
Ambuj