what is C equals 2 pi r?

It's the equation for the circumference  - C - of a circle.......

The circumference =  2 * pi * [the radius of the circle]   →   C = 2 pi r

It looks like they are talking about CIRCLES.   So, "C" stands for Circumference of a circle=2 X r X Pi, where r stands for radius. When you multiply r by 2 it gives you the DIAMETER of a circle. So when you divide C/2r or C/Diameter, you get Pi=3.141592653.