Imagine that white rectangle as a blade that cuts the cylinder as the diagram shows. If you pull the top cylinder off and examine the bottom of that upper piece, then you'll see a circle forms. It's congruent to the circular face of the original cylinder. This is because the cutting plane is parallel to both base faces of the cylinder. Any sort of tilt will make an ellipse form. Keep in mind that any circle is an ellipse, but not vice versa.
Another example of a cross section: cut an orange along its center and notice that a circle (more or less) forms showing the inner part of the orange.
Yet another example of a cross section: Imagine an egyptian pyramid cut from the top most point on downward such that you vertically slice it in half. If you pull away one half, you should see a triangular cross section forms.
