What is it for?
For a general information on constraint programming read Wikipedia article here
Why is it here?
First of all, OpenL tablets uses Constrainer to do some pretty cool stuff with Decision Table validation. Second, Constrainer can benefit from OpenL Tablets unmatched ability to define complex business problems in intuitive business user-friendly way. And sure, Constrainer can use such a nice feature of OpenL as Operator Overloading to express constraints in a natural way.