This red arrow indicates your grain-line, which you should align with the grain of your fabric. Don’t know what grain-line
or grain is? Let us explain. When fabric is made it is woven with threads that go in two different directions. Lengthwise,
which spans the cut edges and crosswise, which spans the selvage edges (the selvage is the band of tightly woven
fabric that runs up either side). The lengthwise direction is the grain of your fabric and what you use to line up your
grain-line.