Very simply, it is a place in which we can evaluate our business rules. Without it, our rules would be stuck 'on paper' and we'd have no way of feeding them into our system.
Rule Engine is a piece of software, which having some knowledge, is able to perform conclusions by using the knowledge.Rule Engines are pluggable software components that separate the business rules from the application code. Rule Engine tells WHAT TO DO at any situation, not HOW TO DO IT.
In technical language of
Drools, we evaluate all the rules in the Rule Engine.
For all the Microsoft Outlook users, it would be very easy to relate to a rule. Its same as described above that when subject contains Jboss Rules then take a specific action. So when the WHEN part of the condition is met, the THEN part of the rule is triggered and actioned.