Cross Sections
A cross section is the shape we get when cutting straight through an object.

It is like a view into the inside of something made by cutting through it.
This is a crosssection of a piece of celery. 
Geometry
In geometry it is the shape made when a solid is cut through by a plane.
Example:
The cross section of this circular cylinder is a circle
We don't draw the rest of the object, just the shape made when you cut through.
Example:
The cross section of a rectangular pyramid is a rectangle
Cross sections are usually parallel to the base like above, but can be in any direction.
Example:
The vertical cross section through the center of this torus is two circles!
And the horizontal cross section is an annulus
More Examples of Cross Sections
Square Prism:  CrossSection: 
Cube:  CrossSection: 
Pentagonal Prism:  CrossSection: 