If you are looking at a ball any ball that is symmetrical, not a foot ball. But more like a ball bearing. That is a spherical entity. Others here have described what the radius is of that spherical ball or entity.
Now if you use a compass and draw a nice circle on a piece of paper, that is the 2 dimensional view and if you draw a straight line from the center point where the compass point was put, to the curve of that circle that line represents the radius.
If you use some thing to measure the ball on the outside and find the largest points by moving the measuring tool around to find them. You simply take that measurement and divide by 2 and you will have the spherical radius.
A sphere every point on its surface is equidistant from its center. And if you cut that sphere in half in any direction you choose, and look at that cut straight on perfectly square to the cut, you will see the 2 d radius that you just used the compass to draw. Is this a simple enough explanation?