A section is a drawing that shows a cut through an object. Many CAD programs will do this for you, but you also nee to know how to draw one of these for yourself so that you can understand the technique behind it.
Start with the plan view and protect the lines down. Next, project the lines from the elevation across to give you the heights. Add the thickness to the object.
In this view I’ve used the same technique to project the lines down from the plan, but this time of drawn a cross-section through a site. Notice how I’ve project of the lines down wherever the contours cross the section line. This is what you need to do to give you the correct site cross-section. Many CAD programs give you the ability to create a site model, and create sections where ever you need them.