Woven fabric is made from threads woven across and down, the warp and weft. When you buy fabric it is cut along the warp and weft, or even ripped along the weft to give straight edges. To cut on the bias you use a line at 45 degrees to the warp and weft. This gives a piece which is stretchy and won't fray so easily. Quilts are edged with bias cut strips so that they stretch a bit and also to give a more durable edge.