What is a geologic cross section?

Taken along a line drawn on a map, it is a diagram of the structure and position of the rock layers present beneath the surface along this line.

Geologists learn how to construct this 3rd dimensional information from a Geologic map,

If you want to know how to get at an ore body hidden underground, you need a Geologist to work out where and how deep to dig. Cross sections will be needed to do this.