A cross section (for 3 dimensional figures) is a 2 dimensional object that is essentially added up to get the figure. In this case, when we cut a horizontal line within the prism, we end up with a rectangle (hence the name rectangular prism). Credits to https://www.mathsisfun.com/geometry/images/pyramid-cross-section.svg for some help with this.
If it was perpendicular to the base, we'll have to note that the shape has a point with a base that is part of the rectangular base, making a triangle
