Constraints are additional parameters to your request.

So when you retrieve data form the database you can add a XPath constraint. 

Example

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 !

https://docs.mendix.com/refguide/xpath-constraints/

Besides Retrieve activities, you can also write Xpath constraints when you use data containers in your pages.

https://docs.mendix.com/refguide/xpath-source/

Best Regards
Filipe Lopes