It's something you might not have noticed until you started sewing. The pattern on a printed fabric is not always lined up with the grain of the fabric (the up-and-down and side-to-side direction of the thread -- the warp and weft). Back when all patterns were woven into fabrics this wasn't a problem because the weave was the pattern. But now with most patterns being printed onto the fabric after it's woven, the 'registration' can be off a little. Or a lot. And that can wreak havoc with your sewing.