Could someone please explain this to me?

I have read this a number of times, but cannot understand what they mean in the first paragraph of step 2. Could someone help me out? I am just drawing a complete blank.

http://www.hgtv.com/hgtv/gl_design_decorative/article/0,,HGTV_3565_1378562,00.html

especially the following lines

"First, he lays the lumber on top of the arc template, then marks the points where the arc and the boards intersect. Then, he draws the radius of the circle through these points"....

Any input greatly appreciated.

thanks,
xman