I believe it's called a slAtted rack (after the slats from each side).
Actually, it's spare slats that I use in the division board feeders as
the diagonal piece of wood mentioned in my previous post.
 
Anyway, a slatted rack goes between the bottom board and the brood
chamber, slatted side up, fat slat towards the front.  It serves a
number of purposes: it reduces drafts into the brood chamber,
it provides clustering room between the brood chamber and bottom board,
some say that bees are less defensive with the slatted racks as there
is a barrier between the brood and the hive entrance, some claim better
hive ventillation and there are probably other claims I have missed.
I use slatted racks on some of my hives, but I am not convinced that
they are "required" equipment.  They're like queen excluders, some
swear by them, some swear at them.  The best write up I have ever seen
about slatted racks is in Eugene Killion's _Honey_In_The_Comb_, where
he swears by them.  However Lloyd Spear (Owner of Ross Rounds, Inc.)
claims that slatted racks are unnecessary (I have never heard Lloyd
swear at anything :)
 
Aaron Morris - thinking the controversy continues!