This article is from the Recreational Figure Skating FAQ, by Karen Bryden with numerous contributions by others.
Hollow or grind refers to the concave surface on the bottom of a
correctly ground blade. A small radius creates edges that will dig
deeply into the ice, while a larger radius digs in less, but glides
more freely. A hollow with a 5/8'' to 3/4'' radius is recommended for
beginners and "all purpose" skates. This hollow will allow you to
sense how a proper edge should feel, and at the same time be forgiving
in things like T-stops. The weight of the skater will also affect how
deeply it should be ground: Usually children will need a deeper hollow
than fully grown adults.
Finally, the width of the blade is yet another factor to consider: A
deep hollow with a 3/8'' or smaller radius will be UNFORGIVING on
freestyle blades, unless you are a child or have a very petite frame.
This type of grind may yields crisp and fast 3-turns, ability to hold
a very deep edge when landing jumps, and allows for fast spins IF you
have them centered. An uncentered spin on deep edges will cause you to
travel. See about traveling. On the other hand, such a small radius
will be ideal for dance or hockey blades: Because these blades are
narrower than freestyle blades, the need a deeper grind to get the
same grip on the ice.
A shallow "figure" hollow with a 1'' or larger radius will require a
more correct lean to prevent skidding and requires more frequent
sharpening, but yields an easy glide and clean tracings.