I know it's probably a stupid noobish question but i've heard this term alot.
the shape of the neck of the guitar on the backside where your fretboard isnt itself
basically an avrage shape neck, it means the back is in the shape of a C or sometimes called a U. V necks are the same but more of a rounded V neck
C-shaped are pretty much a semi-circle. D-shaped necks have a flat spot on the back, and arent too common.
I've got an old semi-hollow from the '60s that has a D-shaped neck (not original, but same era). I love D necks now. I've also got a couple '60s guitars with really fat C necks...don't like those necks much...