URL Parameters is a built-in Data Source in Appfarm for adding variables to be used as parameters of your App url. The use case for this is typical to maintain some App State when e.g. the App is refreshed.
A good example of this is when you have selected a Project from a list of Projects, and navigated to the Project Details view: You would like that same Project to be selected and displayed when you refresh the page. Adding a URL Paramenter for holding the selected Project ID solved this.