The body of influence geometry will refine the mesh (in the next lesson) within the bounds of our body based on the options we want. Since the most interesting things in the flow are happening near the wing, we want to refine the mesh in this area and we do not really need to refine the mesh far from the wing. This is why we are using a body of influence.
We use add frozen here because by default any additional extrusions we create will be automatically added to the first body. When we select "Add Frozen" this body is ignored and creates a separate body from our flow domain body.