Do you know how to find out the grain direction of a piece of paper? Try bending it carefully from the long side or the short side. When you determine which way was easiest to bend, you found the grain direction.
When you are making a book or book cover, it’s preferable to have the grain vertical with the spine and the grain of your paper going in the same direction as your board. This will help prevent your book/box/portfolio from warping, and the pages from curling.