To explain radius or hollow, itsourice.com explains it best:
The radius of hollow can be varied from ¼-inch to 1 ½-inch, however most players will chose a hollow between 3/8-inch and 5/8-inch. Radius of hollow is the most important aspect of skate sharpening to understand. It’s really pretty simple. The bigger the radius, the shallower the hollow on the blade will be. Picture the radius as a wheel. A bigger wheel is going to be closer to flat than a small wheel. That is the impression the wheel leaves on your skate blade.
The depth of hollow changes the feel of the skates on the ice. A deep hollow such as 3/8-inch will give significant “bite” on the edges at the cost of glide. A shallower hollow like 5/8-inch will allow better glide with some loss of edge bite. Thus, if you’re tripping over your edges and landing on your head, the hollow you are using is likely too deep. If your skates are skidding wildly out of control, you probably need to go deeper on your hollow.
– See more at: itcourice.com