That is the bias. When you have a pattern that states "cut on bias" the pattern piece would be placed in a fastion that would be a "diagonal line against fabric grain". Pattern pieces cut in this fashion are sometimes more difficult to sew because when fabric is cut on the bias it gives the cut edge more of a stretch.
bias
BIAS
BIAS
Straight line ending in arrowheads, means "place on straight grain of fabric."
an oblique or diagonal line of direction, esp. across a woven fabric.
a line going diagonally across the grain of fabric
A diagonal is a line that is slanted
The way the fabric is woven gives it different properties in different directions. When you are cutting a pattern you want to get all the pieces on the same "grain", i.e. a piece that will be vertical on your body should not be cut diagonally on the peace of fabric (unless you cut all the pieces diagonally or on the bias). The grain of the fabric is the natural direction of the fabric. The selvage is the edge of the fabric. When you buy a piece of fabric from the roll, you get two cut edges that fray and two factory edges that look different from the rest of the fabric and don't fray. These edges are the selvage. If the grain of the fabric runs from selvage to selvage that means the natural direction of the fabric (that you should use to line up your pattern pieces) runs straight across the fabric from factory edge to factory edge.
For sewing pattern layout, folds are made along the grain line. Lengthwise center fold would be folding the fabric with selvages together. Crosswise centerfold is folding the fabric at the crosswise grain, that is along the weft threads from selvage to selvage.
The diagonal line of a rectangle for example is greater than its length.
Only a line can be diagonal, not a shape
A diagonal is a line so the area of any diagonal must be zero.