Let's understand what a cross section is. By definition, it is a surface or shape exposed by making a straight cut through something, especially at right angles to an axis. Now, what does this mean? Let us take the example of this cone, the shaded portion that you see in the third figure is actually the cross section. Now, this is a surface that we have obtained by cutting this cone using the plane, the low plane that you're seeing, which is actually right angles to this axis, but not all the planes have to be at right angles. Suppose, if somebody has gifted you a ring and you want to design it again, if we know the cross section, we can draw that in a 2d plane and then project it in the 3d environment.

Here you can see the shape of the vast once you know the cross section, I can draw it in the 2d plane and then revolve it to get it Sultan shape. Fusion 360 has some tools with the help of which you can understand these cross sections when let's look at them in the coming video.