It has to do with Report Models. There is an underlying object in SSRS called a Report Model. If you go to the home page in SSRS you'll see a folder for Report Models:
If your goal was to somehow edit the query... that's going to be tough. But there are things you can do
In the Data Set Properties window, click on Query Designer
Here you can add fields by double clicking on them in the Fields area.
In the Data Set Properties window, you can still add parameters, filters, etc.
To get a better understanding of how this all works, follow this tutorial.