Cards stock and paper have a natural grain. Just like wood the fibers lay in a certain direction. When you fold paper and card stock with the grain you'll get a better, cleaner fold ( your card stock won't crack at the folds ).
So how do you know which way the paper's grain is running? Try this tip!
Gently bend a piece of card stock or Designer Series paper horizontally. Open the bend and bend the paper vertically. One direction should feel easier to bend than the other.
If you fold in the direction of the resistance, you will be folding against the grain and get a "rougher' fold. When you fold with the grain of the paper, folding will be easier and you will achieve a smoother fold with no cracks.
material from Stampin' Up! May 2009 publication