A cross section is a 2D drawing of what you see when you cut a 3D form in two. Watch the videos and read the information on this page to get a clearer understanding of how sectional views work.
Try these questions in Khan Academy
Khan Academy - Simple Cross Sections
Engineering Essentials
Sectional Views on Technology Student
See what the sections look like in Geogebra
Geogebra - Sections os 3D shapes