You can find more about parameters here.
You can also check out the Mendix Introduction Course. The module Microflow Basics will explain how parameters work.
In the modeler, there is a bar with all objects you can add in a microflow. The yellow one (see image) adds a parameter to a microflow.
Regards and success,
This is my data source MF.
This is the submicroflow for the source MF.
This is my domain model.
This is the error I am getting while using the data souce MF.