The paper manufacturing process is a continuous production of reels, rolls, or sheets adapted for an end-use. The paper can be cut in the direction of the reel/roll unwinding (machine direction), with the fibers then being parallel to the long side of the sheet. This is referred to as the "long grain" or “machine direction” (MD). If the paper is cut in the other direction (cross direction), it is said to be cut in the "short grain" or “cross direction” (MD) orientation.
Depending on the finishing and binding of the final product, you will need sheets with fibers in the machine or cross direction.
When the paper is to be subsequently folded, it is important to ensure that the folding line is in the long direction of the sheet. If not, folding could damage the fibers, break them, and make them visible at the fold location.