Those are the side views of those different shapes. When you click on that, it will reorient you to the front plane and we can kind of see three different profiles here. All right, we want to control how these profiles loft together, so go to Create Sketch and what I want you to do is create a sketch on the front plane.

So turn off the visibility of this body, just for a moment. If you open up the Sketches, you can see a circle going through a circle to a square but this isn't really what we want, we want to make this a little bit more interesting. I've loaded up the 2-14 Sample File and I would invite you to do so as well. Lofts can create really complex shapes and rails are a way to control the lofts to do what you want them to do. This video introduces the loft, a specific 3D modeling technique, using animations.

Rails are a way of controlling lofts even more. As a self-taught 3D modeler, I found it difficult and frustrating to understand a tool or process without having any context for when it might be used.

Fusion 360 loft tool