Constraints are additional parameters to your request.
So when you retrieve data form the database you can add a XPath constraint.
You want to retrieve all customers which have a brons status.
Then you will retrieve all customers, which have a status = Bronze.
Selected entity for the retriev = Customer
Xpath constraint = [Status = 'Bronze]
Here you have the link for XPath constraints, and how to use XPath in order to constrain data !
Besides Retrieve activities, you can also write Xpath constraints when you use data containers in your pages.